Fix NPE in declarative aggregation execution.

This commit fixes an issue where using a simple return type leads to NPE when the actual aggregation result does not contain any values.

Closes: #3623
Original pull request: #3625.

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

@@ -163,9 +163,9 @@ abstract class AggregationUtils {
 	 * @throws IllegalArgumentException when none of the above rules is met.
 	 */
 	@Nullable
-	static <T> T extractSimpleTypeResult(Document source, Class<T> targetType, MongoConverter converter) {
+	static <T> T extractSimpleTypeResult(@Nullable Document source, Class<T> targetType, MongoConverter converter) {
 
-		if (source.isEmpty()) {
+		if (source == null || source.isEmpty()) {
 			return null;
 		}
 

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

@@ -159,6 +159,14 @@ public class StringBasedAggregationUnitTests {
 		assertThat(executeAggregation("returnCollection").result).isEqualTo(expected);
 	}
 
+	@Test // GH-3623
+	public void returnNullWhenSingleResultIsNotPresent() {
+
+		when(aggregationResults.getMappedResults()).thenReturn(Collections.emptyList());
+
+		assertThat(executeAggregation("simpleReturnType").result).isNull();
+	}
+
 	@Test // DATAMONGO-2153
 	public void returnRawResultType() {
 		assertThat(executeAggregation("returnRawResultType").result).isEqualTo(aggregationResults);
@@ -312,6 +320,9 @@ public class StringBasedAggregationUnitTests {
 
 		@Aggregation(RAW_GROUP_BY_LASTNAME_STRING)
 		Page<Person> invalidPageReturnType(Pageable page);
+
+		@Aggregation(RAW_GROUP_BY_LASTNAME_STRING)
+		String simpleReturnType();
 	}
 
 	static class PersonAggregate {