DATAMONGO-2423 - Polishing.

Update nullability annotations and reduce visibility in tests.

Original pull request: #815.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/index/GeospatialIndex.java

@@ -184,7 +184,6 @@ public class GeospatialIndex implements IndexDefinition {
 		return document;
 	}
 
-	@Nullable
 	public Document getIndexOptions() {
 
 		Document document = new Document();

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/mapping/CachingMongoPersistentProperty.java

@@ -30,7 +30,7 @@ public class CachingMongoPersistentProperty extends BasicMongoPersistentProperty
 
 	private @Nullable Boolean isIdProperty;
 	private @Nullable Boolean isAssociation;
-	private @Nullable boolean dbRefResolved;
+	private boolean dbRefResolved;
 	private @Nullable DBRef dbref;
 	private @Nullable String fieldName;
 	private @Nullable Class<?> fieldType;

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/mapreduce/MapReduceResults.java

@@ -101,6 +101,7 @@ public class MapReduceResults<T> implements Iterable<T> {
 		return outputCollection;
 	}
 
+	@Nullable
 	public Document getRawResults() {
 		return rawResults;
 	}