Asset shelf: Use asset representation for asset shelf BPY methods

Changes the `asset_poll()` and `draw_context_menu()` methods for asset
shelves to use the `AssetRepresentation` type, instead of `AssetHandle`.
The latter should be removed, so it's better to avoid using it in the
asset shelf BPY to avoid future compatibility breakage. This is possible
now with d421ebac5e.

scripts/templates_py/ui_asset_shelf.py

@@ -11,7 +11,7 @@ class MyAssetShelf(bpy.types.AssetShelf):
 
     @classmethod
     def asset_poll(cls, asset):
-        return asset.file_data.id_type in {'MATERIAL', 'OBJECT'}
+        return asset.id_type in {'MATERIAL', 'OBJECT'}
 
 
 def register():