Return nullable strings from the apis that return the bookmark GUIDs

[tarikeshaq]

This is a follow-up to #12300

In the above PR, we added exception handling for all the API places calls to prevent crashes on Errors.

However, in the case a catchable error happens in the retrieval of the bookmarks we return an empty string to the app, which is a smell. We should change those apis to return nullable strings, and have the consuming app handle the null value properly.

The APIs to change are the addItem and addFolder functions in that were modified in the PR mentioned above

┆Issue is synchronized with this Jira Task

[csadilek]

Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1794914

Change performed by the Move to Bugzilla add-on.