[Serializer] (re)document PRESERVE_EMPTY_OBJECTS

Author: 94noni

serializer.rst

@@ -1869,6 +1869,13 @@ to ``true``::
     ]);
     // $jsonContent contains {"name":"Jane Doe"}
 
+Preserving Array Values
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+By default, the Serializer will transform an empty array to `[]`.
+You can change this behavior by setting the ``AbstractObjectNormalizer::PRESERVE_EMPTY_OBJECTS`` context option
+to ``true``, the value would be `{}`.
+
 Handling Uninitialized Properties
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~