Repository: springdoc/springdoc-openapi
Issue: Minimal project with `use-management-port` set causes NPE on api-docs endpoint

**Describe the bug**
In a minimal example, use-management-port is generating NPEs when accessing v3/api-docs.
```
java.lang.NullPointerException: null
	at org.springdoc.core.ActuatorProvider.getApplicationPort(ActuatorProvider.java:185) ~[springdoc-openapi-common-1.5.2.jar:1.5.2]
	at org.springdoc.api.AbstractOpenApiResource.getActuatorURI(AbstractOpenApiResource.java:1061) ~[springdoc-openapi-common-1.5.2.jar:1.5.2]
	at org.springdoc.webmvc.api.OpenApiActuatorResource.getServerUrl(OpenApiActuatorResource.java:152) ~[springdoc-openapi-webmvc-core-1.5.2.jar:1.5.2]
	at org.springdoc.webmvc.api.OpenApiResource.calculateServerUrl(OpenApiResource.java:295) ~[springdoc-openapi-webmvc-core-1.5.2.jar:1.5.2]
	at org.springdoc.webmvc.api.OpenApiResource.openapiJson(OpenApiResource.java:171) ~[springdoc-openapi-webmvc-core-1.5.2.jar:1.5.2]
	at org.springdoc.webmvc.api.OpenApiActuatorResource.openapiJson(OpenApiActuatorResource.java:131) ~[springdoc-openapi-webmvc-core-1.5.2.jar:1.5.2]
```

This appears to be because the ActuatorProvider is trying to access a port from a null `applicationWebServer`. However the previous call's if statement in AbstractOpenApiResource might also be taking the wrong path. It could also be that my example is too minimal and I am missing some configuration.

**To Reproduce**
* Steps to reproduce the behavior:
* Created a simple project with one api endpoint, no groupings.
* Add Springdoc, v3/api-docs returns valid apis.
* Enable use-management-port, and set a management port.
* actuator/openapi produces NPE

What version of spring-boot you are using?
- 2.4.1
What modules and versions of springdoc-openapi are you using?
- 1.5.2

**Expected behavior**
- The same response I get when accessing without using the management port.


Changed files:
springdoc-openapi-common/src/main/java/org/springdoc/core/ActuatorProvider.java

diff --git a/springdoc-openapi-common/src/main/java/org/springdoc/core/ActuatorProvider.java b/springdoc-openapi-common/src/main/java/org/springdoc/core/ActuatorProvider.java
index 89fad675..c246d72f 100644
--- a/springdoc-openapi-common/src/main/java/org/springdoc/core/ActuatorProvider.java
+++ b/springdoc-openapi-common/src/main/java/org/springdoc/core/ActuatorProvider.java
@@ -30,6 +30,7 @@ import org.springdoc.api.AbstractOpenApiResource;
 import org.springframework.boot.actuate.autoconfigure.endpoint.web.WebEndpointProperties;
 import org.springframework.boot.actuate.autoconfigure.web.server.ManagementServerProperties;
 import org.springframework.boot.autoconfigure.web.ServerProperties;
+import org.springframework.boot.web.context.WebServerApplicationContext;
 import org.springframework.boot.web.context.WebServerInitializedEvent;
 import org.springframework.boot.web.server.WebServer;
 import org.springframework.context.ApplicationContext;
@@ -102,12 +103,11 @@ public abstract class ActuatorProvider  implements ApplicationListener<WebServer
 
 	@Override
 	public void onApplicationEvent(WebServerInitializedEvent event) {
-		if ("application".equals(event.getApplicationContext().getId())) {
-			applicationWebServer = event.getWebServer();
-		}
-		else if ("application:management".equals(event.getApplicationContext().getId())) {
+		if (WebServerApplicationContext.hasServerNamespace(event.getApplicationContext(), "management")) {
 			managementApplicationContext = event.getApplicationContext();
 			actuatorWebServer = event.getWebServer();
+		} else {
+			applicationWebServer = event.getWebServer();
 		}
 	}