Merge branch '6.4' into 7.2

* 6.4:
  Tweaks
  [Serializer] (re)document PRESERVE_EMPTY_OBJECTS

Author: javiereguiluz

Diff for: serializer.rst

@@ -1869,6 +1869,14 @@ to ``true``::
     ]);
     // $jsonContent contains {"name":"Jane Doe"}
 
+Preserving Empty Objects
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+By default, the Serializer transforms an empty array to ``[]``. You can change
+this behavior by setting the ``AbstractObjectNormalizer::PRESERVE_EMPTY_OBJECTS``
+context option to ``true``. When the value is an instance of ``\ArrayObject()``,
+the serialized data will be ``{}``.
+
 Handling Uninitialized Properties
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~