Ensure smarty variable editSmartGroupURL is always defined on group edit

[braders]

Overview

Ensure smarty variable editSmartGroupURL is always defined on group edit

Before

When editing a group without an associated smart group URL a warning is thrown (I think this would be a notice below PHP8):

Undefined array key "editSmartGroupURL`

After

The warning is gone.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).