add empty_data to the form HiddenType

Author: osavchenko

reference/forms/types/hidden.rst

@@ -14,7 +14,8 @@ The hidden type represents a hidden input field.
 |             | - `required`_                                                        |
 +-------------+----------------------------------------------------------------------+
 | Inherited   | - `data`_                                                            |
-| options     | - `error_mapping`_                                                   |
+| options     | - `empty_data`_                                                      |
+|             | - `error_mapping`_                                                   |
 |             | - `mapped`_                                                          |
 |             | - `property_path`_                                                   |
 +-------------+----------------------------------------------------------------------+
@@ -49,6 +50,8 @@ These options inherit from the :doc:`FormType </reference/forms/types/form>`:
 
 .. include:: /reference/forms/types/options/data.rst.inc
 
+.. include:: /reference/forms/types/options/empty_data.rst.inc
+
 .. include:: /reference/forms/types/options/error_mapping.rst.inc
 
 .. include:: /reference/forms/types/options/mapped.rst.inc