Fix missing context attribute access in asset shelf polls

When creating a new main window, the `context.object` attribute wouldn't
exist, causing an error message to be printed.

Rather than checking if the attribute extists, query the mode from
context directly (this is always available), rather than querying it
through the active object.

scripts/templates_py/ui_asset_shelf.py

@@ -7,7 +7,7 @@ class MyAssetShelf(bpy.types.AssetShelf):
 
     @classmethod
     def poll(cls, context):
-        return bool(context.object and context.object.mode == 'OBJECT')
+        return context.mode == 'OBJECT'
 
     @classmethod
     def asset_poll(cls, asset):