Fix crash in KAStatsFavouritesModel

Summary:
KAStatsFavouritesModel doesn't initialise a D-pointer for the lifespan
of the KAStatsFavouritesModel, like the name implies.

It is set in initForClient.

If the activity changes before we have a client ID, there's no point
doing anything and trying to use the clientID will crash.

BUG: 386439

Test Plan: None.

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: anthonyfieroni, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8608