Address nullness warning from future version of NullAway

Signed-off-by: Manu Sridharan <msridhar@gmail.com>

Author: msridhar

spring-core/src/main/java/org/springframework/core/annotation/AnnotatedElementUtils.java

@@ -823,7 +823,7 @@ private static MergedAnnotations findRepeatableAnnotations(AnnotatedElement elem
 		return MergedAnnotations.from(element, SearchStrategy.TYPE_HIERARCHY, repeatableContainers);
 	}
 
-	private static @Nullable MultiValueMap<String, Object> nullIfEmpty(MultiValueMap<String, Object> map) {
+	private static @Nullable MultiValueMap<String, @Nullable Object> nullIfEmpty(MultiValueMap<String, @Nullable Object> map) {
 		return (map.isEmpty() ? null : map);
 	}