Fix windows like kickoff from disappearing

When trying to launch kickoff (start menu) or the add widgets from
Plasma, they disappeared immediately after appearing, making them
completely unusable.

This regression was traced back to commit
bf0a241c2462072fbc52012784fa51b9b42a1baf.

In Client::setOnActivities, if newActivitiesList was empty, after
joining it and splitting it back, it would contain an empty string, i.e.
it would have a different size than before. This eventually caused the problems.

REVIEW: 105016