DATAMONGO-570 - Guard against null values in references.

QueryMapper does not try to convert null values into DBRef objects anymore but returns the plain null value as is.

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

@@ -263,7 +263,7 @@ public class QueryMapper {
 			return result;
 		}
 
-		return source instanceof DBRef ? source : converter.toDBRef(source, property);
+		return source == null || source instanceof DBRef ? source : converter.toDBRef(source, property);
 	}
 
 	/**

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/QueryMapperUnitTests.java

@@ -335,6 +335,18 @@ public class QueryMapperUnitTests {
 		assertThat(reference.containsField("$in"), is(true));
 	}
 
+	/**
+	 * @see DATAMONGO-570
+	 */
+	@Test
+	public void correctlyConvertsNullReference() {
+
+		Query query = query(where("reference").is(null));
+		DBObject object = mapper.getMappedObject(query.getQueryObject(), context.getPersistentEntity(WithDBRef.class));
+
+		assertThat(object.get("reference"), is(nullValue()));
+	}
+
 	class IdWrapper {
 		Object id;
 	}