Applying 0120_scope_file_stores_data migration fails if there is a scope with no workspaces

In confidential issue #702, @helmutg reported a failure to apply 0120_scope_file_stores_data on an instance where there is currently a scope with no workspaces, because the migration attempts to create a FileStoreInScope row with a null file_store. In such cases, we should leave the scope in question with no related file stores, since that's equivalent to the previous setup.