Repository: quarkusio/quarkus
Issue: java.lang.IllegalArgumentException: Unable to resolve locale: c.u-US

When I try to build the jar on a server I got  the next error
```
[INFO] --- quarkus-maven-plugin:1.7.0.Final:build (default) @ quarkus-cloud ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  32.095 s
[INFO] Finished at: 2020-08-17T22:05:49Z
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal io.quarkus:quarkus-maven-plugin:1.7.0.Final:build (default) on project quarkus-cloud: Failed to build quarkus application: java.lang.IllegalArgumentException: Unable to resolve locale: c.u-US -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
```

**Additional context**
IBM cloud toolchain

On my computer works just fine: macos 10.15.6 (19G2021)



Changed files:
core/runtime/src/main/java/io/quarkus/runtime/configuration/LocaleConverter.java
core/runtime/src/test/java/io/quarkus/runtime/configuration/LocaleConverterTest.java

diff --git a/core/runtime/src/main/java/io/quarkus/runtime/configuration/LocaleConverter.java b/core/runtime/src/main/java/io/quarkus/runtime/configuration/LocaleConverter.java
index 304668aa737..cab0ff13948 100644
--- a/core/runtime/src/main/java/io/quarkus/runtime/configuration/LocaleConverter.java
+++ b/core/runtime/src/main/java/io/quarkus/runtime/configuration/LocaleConverter.java
@@ -30,7 +30,7 @@ public Locale convert(final String value) {
         }
 
         Locale locale = Locale.forLanguageTag(NORMALIZE_LOCALE_PATTERN.matcher(localeValue).replaceAll("-"));
-        if (locale.getLanguage() == null || locale.getLanguage().isEmpty()) {
+        if (locale != Locale.ROOT && (locale.getLanguage() == null || locale.getLanguage().isEmpty())) {
             throw new IllegalArgumentException("Unable to resolve locale: " + value);
         }
 
diff --git a/core/runtime/src/test/java/io/quarkus/runtime/configuration/LocaleConverterTest.java b/core/runtime/src/test/java/io/quarkus/runtime/configuration/LocaleConverterTest.java
index 0c924c9d781..87d61ffe46c 100644
--- a/core/runtime/src/test/java/io/quarkus/runtime/configuration/LocaleConverterTest.java
+++ b/core/runtime/src/test/java/io/quarkus/runtime/configuration/LocaleConverterTest.java
@@ -1,6 +1,8 @@
 package io.quarkus.runtime.configuration;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
 
 import java.util.Locale;
 
@@ -19,8 +21,23 @@ public void testStandardLocale() {
     @Test
     public void testUnderscoreLocale() {
         LocaleConverter localeConverter = new LocaleConverter();
-        Locale locale = localeConverter.convert("fr-FR");
+        Locale locale = localeConverter.convert("fr_FR");
         assertEquals("fr", locale.getLanguage());
         assertEquals("FR", locale.getCountry());
     }
+
+    @Test
+    public void testCLocale() {
+        LocaleConverter localeConverter = new LocaleConverter();
+        Locale locale = localeConverter.convert("c.u-US");
+        assertSame(Locale.ROOT, locale);
+    }
+
+    @Test
+    public void testEmptyLocale() {
+        LocaleConverter localeConverter = new LocaleConverter();
+        Locale locale = localeConverter.convert("");
+        assertNull(locale);
+    }
+
 }