Use shared InjectionMetadata instance for beans skipped by AutowiredAnnotationBeanPostProcessor

[philwebb]

Currently AutowiredAnnotationBeanPostProcessor.buildAutowiringMetadata creates a new InjectionMetadata instance with an empty collection for beans that have no relevant annotations. It looks like the clazz parameter never escapes from the class so we could probably create a special shared InjectionMetadata.EMPTY instance to return in such cases.