Update 'attributes' list type to enable nulls in header style dropdown config. (#1631)

In `select_header_style_dropdown_button_configurations.dart`, the 'attributes' list type has been modified. Now it accepts 'Attribute<int?>' instead of 'Attribute<int>'. This change allows for custom dropdown attributes with a nullable level.

Co-authored-by: Sobhan Moradi <sobhan.moradi@getflip.com>

lib/src/models/config/toolbar/buttons/select_header_style_dropdown_button_configurations.dart

@@ -49,7 +49,7 @@ class QuillToolbarSelectHeaderStyleDropdownButtonOptions
   ///   Attribute.header,
   /// ]
   /// ```
-  final List<Attribute<int>>? attributes;
+  final List<Attribute<int?>>? attributes;
 
   QuillToolbarSelectHeaderStyleDropdownButtonOptions copyWith({
     ValueChanged<String>? onSelected,