allow to reset hydration setting for Aggregation

lib/Doctrine/ODM/MongoDB/Aggregation/Builder.php

@@ -341,7 +341,7 @@ public function group(): Stage\Group
     /**
      * Set which class to use when hydrating results as document class instances.
      */
-    public function hydrate(string $className): self
+    public function hydrate(?string $className): self
     {
         $this->hydrationClass = $className;
 

tests/Doctrine/ODM/MongoDB/Tests/Aggregation/BuilderTest.php

@@ -16,6 +16,7 @@
 use Documents\CmsComment;
 use Documents\GuestServer;
 use Documents\Tag;
+use MongoDB\BSON\ObjectId;
 use MongoDB\BSON\UTCDateTime;
 
 use function array_keys;
@@ -199,6 +200,34 @@ public function testAggregationBuilder()
         $this->assertSame(3, $results[0]->numPosts);
     }
 
+    public function testAggregationBuilderResetHydration(): void
+    {
+        $this->insertTestData();
+
+        $builder = $this->dm->createAggregationBuilder(BlogPost::class)->hydrate(BlogTagAggregation::class);
+
+        $resultCursor = $builder
+            ->hydrate(null)
+            ->unwind('$tags')
+            ->group()
+            ->field('id')
+            ->expression('$tags')
+            ->field('numPosts')
+            ->sum(1)
+            ->sort('numPosts', 'desc')
+            ->getAggregation()
+            ->getIterator();
+
+        $this->assertInstanceOf(Iterator::class, $resultCursor);
+
+        $results = $resultCursor->toArray();
+        $this->assertCount(2, $results);
+        $this->assertIsArray($results[0]);
+        $this->assertInstanceOf(ObjectId::class, $results[0]['_id']['$id']);
+        $this->assertSame('Tag', $results[0]['_id']['$ref']);
+        $this->assertSame(3, $results[0]['numPosts']);
+    }
+
     public function testGetAggregation()
     {
         $this->insertTestData();