Detect accidental misconfiguration of @JsonMixin #33992
Description
@JsonMixin exposes both a value and a type attribute, the latter being an alias of the former, which means that both attributes define an empty default value.
JsonMixinModuleEntries' registerMixinClass(…) inspects the attributes and only registers a mixin if the attribute is set. This means that a sole @JsonMixin annotation on a class will be found but dropped in that purpose. While this is a pretty obvious mistake, as one does not define which class to apply the mixin for, it can easily be overseen and we could actually detect that misconfiguration. It would be nice if ….registerMixinClass(…) could reject an empty @JsonMixin annotation to avoid this oversight.