Update `AnnotationsPropertySource` to ensure that `null` results from
`findMergedAnnotation` are not added to the annotation list.

Prior to this commit, if `findMergedAnnotation` returned `null` then
`AnnotationsPropertySource.collectProperties` would throw an NPE.
Although `findMergedAnnotation` should never return `null`, we're
best off being defensive so that bugs such as SPR-17495 won't cause
problems.

Closes gh-15175