Never return null from AnnotationMetadata.getMetaAnnotationTypes

Issue: SPR-17046

(cherry picked from commit cacd14c)

Author: jhoeller

spring-core/src/main/java/org/springframework/core/type/classreading/AnnotationMetadataReadingVisitor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 package org.springframework.core.type.classreading;
 
 import java.util.Collection;
+import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
@@ -98,7 +99,8 @@ public Set<String> getAnnotationTypes() {
 
 	@Override
 	public Set<String> getMetaAnnotationTypes(String annotationName) {
-		return this.metaAnnotationMap.get(annotationName);
+		Set<String> metaAnnotationTypes = this.metaAnnotationMap.get(annotationName);
+		return (metaAnnotationTypes != null ? metaAnnotationTypes : Collections.emptySet());
 	}
 
 	@Override