Polishing.

Use ObjectUtils for empty check.

See #3623
Original pull request: #3625.

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

@@ -33,6 +33,7 @@ import org.springframework.data.repository.query.QueryMethodEvaluationContextPro
 import org.springframework.expression.ExpressionParser;
 import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
 /**
@@ -165,7 +166,7 @@ abstract class AggregationUtils {
 	@Nullable
 	static <T> T extractSimpleTypeResult(@Nullable Document source, Class<T> targetType, MongoConverter converter) {
 
-		if (source == null || source.isEmpty()) {
+		if (ObjectUtils.isEmpty(source)) {
 			return null;
 		}