Fix Firefox profile location lookup after location has changed

Summary:
When the profile has initially been looked up, the location is saved to
the global config. Afterwards, the location is never updated even if
the profile has been replaced, due to an inverted exits() check.

Test Plan:
1. Check dbfile in kdeglobals is pointing to a valid profile
2a. Change the dbfile setting - or -
2b. Remove the existing profile and create a new one
3. Restart krunner - it should fix the config entry/use the new profile

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15254