DATAMONGO-2542 - Polishing.

Fix nullable annotation.

Original pull request: #863.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java

@@ -1080,7 +1080,7 @@ public class QueryMapper {
 		 */
 		@Nullable
 		private PersistentPropertyPath<MongoPersistentProperty> getPath(String pathExpression,
-				MongoPersistentProperty sourceProperty) {
+				@Nullable MongoPersistentProperty sourceProperty) {
 
 			String rawPath = removePlaceholders(POSITIONAL_OPERATOR,
 					removePlaceholders(DOT_POSITIONAL_PATTERN, pathExpression));