fix displaying empty_data option for hidden field

Author: osavchenko

reference/forms/types/hidden.rst

@@ -51,6 +51,12 @@ 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
+    :end-before: DEFAULT_PLACEHOLDER
+
+The default value is ``''`` (the empty string).
+
+.. include:: /reference/forms/types/options/empty_data.rst.inc
+    :start-after: DEFAULT_PLACEHOLDER
 
 .. include:: /reference/forms/types/options/error_mapping.rst.inc