Fix introspector chaining in JaxbAnnotationIntrospector.hasRequiredMarker()

[vojtechhabarta]

Let's say I have a property marked with @JsonProperty(required = true).
I also used objectMapper.registerModules(ObjectMapper.findModules(classLoader)) to register modules but now the property is not considered "required".

That's because there is a chain of JaxbAnnotationIntrospector and JacksonAnnotationIntrospector (in this order) and JaxbAnnotationIntrospector.hasRequiredMarker() returns Boolean.FALSE so JacksonAnnotationIntrospector is not called (in AnnotationIntrospectorPair).

This PR fixes this case, JaxbAnnotationIntrospector.hasRequiredMarker() now returns null so property can be processed by subsequent introspector.

This PR also changes that JaxbAnnotationIntrospector.hasRequiredMarker() now authoritatively returns Boolean.FALSE when annotation is present. This behavior is consistent with JacksonAnnotationIntrospector.

[cowtowncoder]

Looking at code, makes sense. Looks like return values were switched around.
The original concern was probably due to false being the default (since annotation values do not have "undefined" or null possibility). But returning Boolean.FALSE as fallback is wrong no matter what.

One practical note: since I want this to be in 2.9.3, I'll manually merge changes (master is for 3.0.0).