Protect against NPE when collecting annotations

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

Author: philwebb

spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/properties/AnnotationsPropertySource.java

@@ -90,8 +90,11 @@ private List<Annotation> getMergedAnnotations(Class<?> root, Class<?> source) {
 		if (annotations != null) {
 			for (Annotation annotation : annotations) {
 				if (!AnnotationUtils.isInJavaLangAnnotationPackage(annotation)) {
-					mergedAnnotations
-							.add(findMergedAnnotation(root, annotation.annotationType()));
+					Annotation mergedAnnotation = findMergedAnnotation(root,
+							annotation.annotationType());
+					if (mergedAnnotation != null) {
+						mergedAnnotations.add(mergedAnnotation);
+					}
 				}
 			}
 		}