Show the "Nothing to export" snack when trying to export rules with empty list of User rules

[Versty]

Issue Details

Currently the context menu option "Export" on the User rules screen (Protection -> Ad blocking -> User rules) can be used as usual even if the list of rules is empty, resulting in the creation of an empty file in the device storage.

Proposed solution

Show the "Nothing to export" snack instead of opening the file system to save an empty file.

Alternative solution

Does not display the "Export" option in the context menu if the list of user rules is empty.

Additional info

The same issue is applicable to User rules from DNS Protection module.