Address nullness warning from upcoming version of NullAway

[msridhar]

NullAway will emit these warnings once uber/NullAway#1499 lands:

spring-core/src/main/java/org/springframework/core/annotation/AnnotatedElementUtils.java:520: error: [NullAway] referenced method returns @Nullable MultiValueMap<String, Object>, but functional interface method returns @Nullable MultiValueMap<String, @Nullable Object>, which has mismatched type parameter nullability
                                .collect(MergedAnnotationCollectors.toMultiValueMap(AnnotatedElementUtils::nullIfEmpty, adaptations));
                                                                                    ^
    (see http://t.uber.com/nullaway )
spring-core/src/main/java/org/springframework/core/annotation/AnnotatedElementUtils.java:520: error: [NullAway] parameter type of referenced method is MultiValueMap<String, Object>, but parameter in functional interface method has type MultiValueMap<String, @Nullable Object>, which has mismatched type parameter nullability
                                .collect(MergedAnnotationCollectors.toMultiValueMap(AnnotatedElementUtils::nullIfEmpty, adaptations));
                                                                                    ^

This PR updates the annotations on nullIfEmpty to address these warnings.

[msridhar]

FYI @sdeleuze