gnome-shell: extension fails to install if no "schemas" folder present

Gnome-shell extensions fail to install if the extension does not have a 'schemas' folder. I can replicate this behaviour when installing from extensions.gnome.org or gnome-shell-extension-manager. By adding an empty schema's folder the extension installs correctly.

I have checked the gjs extension documentation[1] and afaict the 'schemas' folder is not listed as required.

Relevant logs when installing the weatheroclock[2] extension:

```
Mar 28 12:38:07 eart gnome-shell[1444369]: Archive: /tmp/HNKP21.shell-extension.zip
Mar 28 12:38:07 eart gnome-shell[1444369]: inflating: /<email address hidden>/extension.js
Mar 28 12:38:07 eart gnome-shell[1444369]: inflating: /<email address hidden>/stylesheet.css
Mar 28 12:38:07 eart gnome-shell[1444369]: extracting: /<email address hidden>/metadata.json
Mar 28 12:38:07 eart gnome-shell[1423521]: Error while installing <email address hidden>: Error when getting information for file “/<email address hidden>/schemas”: No such file or directory
Mar 28 12:38:07 eart extension-manag[1426090]: GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._g_2dio_2derror_2dquark.Code36: GDBus.Error:org.gnome.Shell.ExtensionError: Error when getting information for file “/<email address hidden>/schemas”: No such file or directory

```

[1] https://gjs.guide/extensions/overview/anatomy.html#metadata-json-required
[2] https://extensions.gnome.org/extension/5470/weather-oclock/