minor #20877 [Serializer] (re)document PRESERVE_EMPTY_OBJECTS (94noni)

This PR was submitted for the 7.2 branch but it was squashed and merged into the 6.4 branch instead.

Discussion
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[Serializer] (re)document PRESERVE_EMPTY_OBJECTS

👋🏻
when implementing a web hook, I had to serialize en empty array as object, and found it was wrong (empty list not empty object)

```php
// json without flag
'data' => [],
"data":[],

// json with flag
'data' => new \ArrayObject(),
"data":{},

```
this PR (re)adds this feature as I thinks its useful to have it directly showcased in the main doc
weirdly it was present in v5.4 #16977

Commits
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ce28629 [Serializer] (re)document PRESERVE_EMPTY_OBJECTS

Author: javiereguiluz

serializer.rst

@@ -1571,6 +1571,13 @@ to ``true``::
     ]);
     // $jsonContent contains {"name":"Jane Doe"}
 
+Preserving Empty Objetcs
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+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``, when the value is `\ArrayObject()` the serialization would be `{}`.
+
 Handling Uninitialized Properties
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~