fix: null reference on gift photo upload (#5547)

Attempting to connect a photo to a gift, either an already existing one
or a freshly uploaded one, the AJAX call would return a 500 server
error. The error states of a `Attempt to read property "id" on null` in
`app\Http\Resources\Gift/Gift` line 24.

This is causes by attempting to use a void result of
`AssociatePhotoToGift` service in `ApiGiftController::associate()`
method.

Fixed by returning the actual `$gift` variable worked on, similar to the
related services.

This does fix the *stacktrace* in #5516, but not the issue itself. The
issue there is likely unrelated to the stacktrace.

Fixes #5397

app/Services/Contact/Gift/AssociatePhotoToGift.php

@@ -40,5 +40,7 @@ public function execute(array $data)
         $gift->contact->throwInactive();
 
         $gift->photos()->syncWithoutDetaching([$photo->id]);
+
+        return $gift;
     }
 }

tests/Unit/Services/Contact/Gift/AssociateGiftToPhotoTest.php

@@ -22,12 +22,15 @@ public function it_associates_a_photo_to_a_gift()
             'account_id' => $gift->account_id,
         ]);
 
-        app(AssociatePhotoToGift::class)->execute([
+        $giftUpdated = app(AssociatePhotoToGift::class)->execute([
             'account_id' => $gift->account_id,
             'gift_id' => $gift->id,
             'photo_id' => $photo->id,
         ]);
 
+        $this->assertInstanceOf(Gift::class, $giftUpdated);
+        $this->assertEquals($gift->id, $giftUpdated->id);
+
         $this->assertDatabaseHas('gift_photo', [
             'gift_id' => $gift->id,
             'photo_id' => $photo->id,