Support returning null in ParameterCustomizer

[martin-tarjanyi]

Is your feature request related to a problem? Please describe.

My use case is that I'd like to hide certain operation parameters based on an annotation and an environment specific condition. I've successfully done this with OperationCustomizer and PropertyCustomizer without any issues but unfortunately in case of ParameterCustomizer this results in a NullPointerException.

Describe the solution you'd like
ParameterCustomizer to support returning null values and omit the parameter from the OpenAPI JSON if customizer returns null

Describe alternatives you've considered
I considered setting some property with a specific value in Parameter and later filter it out in the OperationCustomizer. This works but a bit ugly.

Additional context
Here is my customizer...

  @Bean
  fun parameterEnvironmentCustomizer(): ParameterCustomizer =
    ParameterCustomizer { model, methodParameter ->
      if (methodParameter.parameterAnnotations.any { it.annotationClass == InDevelopmentProperty::class } &&
        !properties.publishInDevelopmentProperties
      ) {
        null
      } else {
        model
      }
    }

...causing this exception:

Caused by: java.lang.NullPointerException: Cannot invoke "io.swagger.v3.oas.models.parameters.Parameter.getName()" because "parameter" is null
	at org.springdoc.core.models.ParameterId.<init>(ParameterId.java:57)
	at java.base/java.util.stream.Collectors.lambda$toMap$68(Collectors.java:1673)
	at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
	at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1625)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
	at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:921)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:682)
	at org.springdoc.core.service.AbstractRequestService.getParameterLinkedHashMap(AbstractRequestService.java:388)
	at org.springdoc.core.service.AbstractRequestService.build(AbstractRequestService.java:351)
	at org.springdoc.api.AbstractOpenApiResource.calculatePath(AbstractOpenApiResource.java:496)
	at org.springdoc.api.AbstractOpenApiResource.calculatePath(AbstractOpenApiResource.java:676)
	at org.springdoc.webmvc.api.OpenApiResource.lambda$calculatePath$11(OpenApiResource.java:219)
	at java.base/java.util.Optional.ifPresent(Optional.java:178)
	at org.springdoc.webmvc.api.OpenApiResource.calculatePath(OpenApiResource.java:200)
	at org.springdoc.webmvc.api.OpenApiResource.lambda$getPaths$2(OpenApiResource.java:170)
	at java.base/java.util.Optional.ifPresent(Optional.java:178)
	at org.springdoc.webmvc.api.OpenApiResource.getPaths(OpenApiResource.java:149)
	at org.springdoc.api.AbstractOpenApiResource.getOpenApi(AbstractOpenApiResource.java:353)
	at org.springdoc.webmvc.api.OpenApiResource.openapiJson(OpenApiResource.java:124)
	at org.springdoc.webmvc.api.OpenApiWebMvcResource.openapiJson(OpenApiWebMvcResource.java:114)
	at org.springdoc.webmvc.api.MultipleOpenApiWebMvcResource.openapiJson(MultipleOpenApiWebMvcResource.java:95)