Repository: hapifhir/org.hl7.fhir.core
Issue: NullPointerException while comparing profiles

### Problem Description
The validator throws an exception while comparing profiles, which constraint meta.profile to some fixed values.

### Steps to reproduce
Call the CLI version of validator as follows:

`java -jar validator_cli.jar -compare -dest "c:\\Dev\\fhir validator\\compare_results" -version 4.0.1 -ig kbv.ita.for#1.1.0 ig kbv.basis#1.3.0 -left "https://fhir.kbv.de/StructureDefinition/KBV_PR_FOR_Patient" -right "https://fhir.kbv.de/StructureDefinition/KBV_PR_Base_Patient"`

Output:

FHIR Validation tool Version 5.6.87 (Git# 6d8cd59f1ee0). Built 2022-12-14T03:25:37.265Z (60 hours old)
  Java:   11.0.14.1 from C:\\Program Files\\OpenJDK\\openjdk-11.0.14.1_1 on amd64 (64bit). 8136MB available

WARNING: Default file encoding is windows-1252 which may cause unexpected results.
         To fix this issue, run this program with the parameter '-Dfile.encoding=UTF-8'
         Future releases may not be able to run at all with encoding windows-1252

Valid destination directory provided: "c:\\Dev\\fhir validator\\compare_results")
  Paths:  Current = c:\\Dev\\fhir validator, Package Cache = C:\\Users\\...\\.fhir\\packages
  Params: -compare -dest "c:\\Dev\\fhir validator\\compare_results" -version 4.0.1 -ig kbv.ita.for#1.1.0 ig kbv.basis#1.3.0 -left https://fhir.kbv.de/StructureDefinition/KBV_PR_FOR_Patient -right https://fhir.kbv.de/StructureDefinition/KBV_PR_Base_Patient
  Load FHIR v4.0 from hl7.fhir.r4.core#4.0.1.: - 4576 resources (00:05.145)
  Load hl7.terminology#4.0.0 - 4164 resources (00:02.725)
  Load R5 Extensions - 127 resources (00:02.379)
  Terminology server http://tx.fhir.org - Version 2.0.14 (00:01.121)
  Load de.basisprofil.r4#1.3.2 - 177 resources (00:00.219)
  Load KBV.Basis#1.3.0 - 146 resources (00:01.981)
  Load kbv.ita.for#1.1.0 - 20 resources (00:00.099)
  Package Summary: [hl7.fhir.r4.core#4.0.1, hl7.fhir.xver-extensions#0.0.11, hl7.terminology#4.0.0, de.basisprofil.r4#1.3.2, KBV.Basis#1.3.0, kbv.ita.for#1.1.0]
  Get set...  go (00:01.393)
  Load hl7.fhir.pubpack#0.1.4 - 0 resources (00:00.718)
Comparing StructureDefinitions https://fhir.kbv.de/StructureDefinition/KBV_PR_FOR_Patient to https://fhir.kbv.de/StructureDefinition/KBV_PR_Base_Patient
Generating output to c:\\Dev\\fhir validator\\compare_results...
Exception rendering https://fhir.kbv.de/StructureDefinition/KBV_PR_FOR_Patient|1.1.0||https://fhir.kbv.de/StructureDefinition/KBV_PR_Base_Patient|1.3.0: null
java.lang.NullPointerException
        at org.hl7.fhir.r5.conformance.ProfileUtilities.generateDescription(ProfileUtilities.java:4151)
        at org.hl7.fhir.r5.conformance.ProfileUtilities.generateDescription(ProfileUtilities.java:3882)
        at org.hl7.fhir.r5.conformance.ProfileUtilities.genElementCells(ProfileUtilities.java:3575)
        at org.hl7.fhir.r5.comparison.ProfileComparer.genElementComp(ProfileComparer.java:1002)
        at org.hl7.fhir.r5.comparison.ProfileComparer.genElementComp(ProfileComparer.java:1014)
        at org.hl7.fhir.r5.comparison.ProfileComparer.genElementComp(ProfileComparer.java:1014)
        at org.hl7.fhir.r5.comparison.ProfileComparer.renderStructure(ProfileComparer.java:937)
        at org.hl7.fhir.r5.comparison.ComparisonRenderer.renderProfile(ComparisonRenderer.java:224)
        at org.hl7.fhir.r5.comparison.ComparisonRenderer.renderComparison(ComparisonRenderer.java:144)
        at org.hl7.fhir.r5.comparison.ComparisonRenderer.processList(ComparisonRenderer.java:111)
        at org.hl7.fhir.r5.comparison.ComparisonRenderer.render(ComparisonRenderer.java:87)
        at org.hl7.fhir.validation.cli.services.ComparisonService.compareStructureDefinitions(ComparisonService.java:74)
        at org.hl7.fhir.validation.cli.services.ComparisonService.doLeftRightComparison(ComparisonService.java:37)
        at org.hl7.fhir.validation.ValidatorCli.doLeftRightComparison(ValidatorCli.java:265)
        at org.hl7.fhir.validation.ValidatorCli.main(ValidatorCli.java:131)
Done

Details of comparison in file:///C:/Dev/fhir%20validator/compare_results/sd-KBV-PR-FOR-Patient-KBV-PR-Base-Patient.html do not exist

### Expected output

```
FHIR Validation tool Version 5.6.89-SNAPSHOT (Git# aaa5c9d6a409). Built 2022-12-16T16:31:11.232+01:00 (8 mins old)
  Java:   11.0.14.1 from C:\\Program Files\\OpenJDK\\openjdk-11.0.14.1_1 on amd64 (64bit). 8136MB available

WARNING: Default file encoding is windows-1252 which may cause unexpected results.
         To fix this issue, run this program with the parameter '-Dfile.encoding=UTF-8'
         Future releases may not be able to run at all with encoding windows-1252

Valid destination directory provided: "c:\\Dev\\fhir validator\\compare_results")
  Paths:  Current = c:\\Dev\\fhir validator, Package Cache = C:\\Users\\...\\.fhir\\packages
  Params: -compare -dest "c:\\Dev\\fhir validator\\compare_results" -version 4.0.1 -ig kbv.ita.for#1.1.0 ig kbv.basis#1.1.3 -left https://fhir.kbv.de/StructureDefinition/KBV_PR_FOR_Patient -right https://fhir.kbv.de/StructureDefinition/KBV_PR_Base_Patient
  Load FHIR v4.0 from hl7.fhir.r4.core#4.0.1.: - 4576 resources (00:04.293)
  Load hl7.terminology#4.0.0 - 4164 resources (00:01.632)
  Load R5 Extensions - 127 resources (00:02.129)
  Terminology server http://tx.fhir.org - Version 2.0.14 (00:01.177)
  Load de.basisprofil.r4#1.3.2 - 177 resources (00:00.211)
  Load KBV.Basis#1.3.0 - 146 resources (00:01.896)
  Load kbv.ita.for#1.1.0 - 20 resources (00:00.092)
  Package Summary: [hl7.fhir.r4.core#4.0.1, hl7.fhir.xver-extensions#0.0.11, hl7.terminology#4.0.0, de.basisprofil.r4#1.3.2, KBV.Basis#1.3.0, kbv.ita.for#1.1.0]
  Get set...  go (00:01.515)
  Load hl7.fhir.pubpack#0.1.4 - 0 resources (00:00.753)
Comparing StructureDefinitions https://fhir.kbv.de/StructureDefinition/KBV_PR_FOR_Patient to https://fhir.kbv.de/StructureDefinition/KBV_PR_Base_Patient
Generating output to c:\\Dev\\fhir validator\\compare_results...
Done
```
Comparison details at file:///C:/Dev/fhir%20validator/compare_results/sd-KBV-PR-FOR-Patient-KBV-PR-Base-Patient.html should exist

Changed files:
org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/conformance/ProfileUtilities.java

diff --git a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/conformance/ProfileUtilities.java b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/conformance/ProfileUtilities.java
index 89d0e134a..151a12ad5 100644
--- a/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/conformance/ProfileUtilities.java
+++ b/org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/conformance/ProfileUtilities.java
@@ -4147,7 +4147,7 @@ public class ProfileUtilities extends TranslatingUtilities {
             if (!useTableForFixedValues || !allowSubRows || definition.getFixed().isPrimitive()) {
               String s = buildJson(definition.getFixed());
               String link = null;
-              if (Utilities.isAbsoluteUrl(s))
+              if (Utilities.isAbsoluteUrl(s) && pkp != null)
                 link = pkp.getLinkForUrl(corePath, s);
               c.getPieces().add(checkForNoChange(definition.getFixed(), gen.new Piece(link, s, null).addStyle("color: darkgreen")));
             } else {