DATACMNS-542 - Be more lenient about simplified way to customize repository base classes.

We now leniently return null for the repository base class name from the AnnotationRepositoryConfigurationSource to make sure Spring Data modules compile against Spring Data Commons 1.10 continue to work on 1.11 without having the annotation attribute added.

src/main/java/org/springframework/data/repository/config/AnnotationRepositoryConfigurationSource.java

@@ -221,6 +221,10 @@ public class AnnotationRepositoryConfigurationSource extends RepositoryConfigura
 	@Override
 	public String getRepositoryBaseClassName() {
 
+		if (!attributes.containsKey(REPOSITORY_BASE_CLASS)) {
+			return null;
+		}
+
 		Class<? extends Object> repositoryBaseClass = attributes.getClass(REPOSITORY_BASE_CLASS);
 		return DefaultRepositoryBaseClass.class.equals(repositoryBaseClass) ? null : repositoryBaseClass.getName();
 	}