Polishing.

Mark method potentially returning null as such and remove unused imports.

See #4404
Original pull request: #4412

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/query/MongoQueryCreator.java

@@ -52,6 +52,7 @@ import org.springframework.data.repository.query.parser.Part.IgnoreCaseType;
 import org.springframework.data.repository.query.parser.Part.Type;
 import org.springframework.data.repository.query.parser.PartTree;
 import org.springframework.data.util.Streamable;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
@@ -352,6 +353,7 @@ class MongoQueryCreator extends AbstractQueryCreator<Query, Criteria> {
 	 * @param part
 	 * @return the regex options or {@literal null}.
 	 */
+	@Nullable
 	private String toRegexOptions(Part part) {
 
 		String regexOptions = null;

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/query/MongoQueryCreatorUnitTests.java

@@ -15,7 +15,6 @@
  */
 package org.springframework.data.mongodb.repository.query;
 
-import static org.mockito.Mockito.*;
 import static org.springframework.data.mongodb.core.query.Criteria.*;
 import static org.springframework.data.mongodb.core.query.Query.*;
 import static org.springframework.data.mongodb.repository.query.StubParameterAccessor.*;