Avoid shadowing variable name + check before conversion

If the docs are to be believed, the settings are meant to have disabled extensions as dicts: https://jupyterlab.readthedocs.io/en/stable/user/directories.html#labconfig-directories

This prevents disabling an extension if the value is `false`.

jupyterlab_server/config.py

@@ -96,8 +96,8 @@ def get_page_config(labextensions_path, app_settings_dir=None, logger=None):
 
             # Convert lists to dicts
             for key in [disabled_key, "deferredExtensions"]:
-                if key in data:
-                    data[key] = dict((key, True) for key in data[key])
+                if key in data and not isinstance(data[key], dict):
+                    data[key] = dict((k, True) for k in data[key])
 
             recursive_update(page_config, data)