[Task Manager] Fix URL comparison

Summary:
You cannot compare to null but to empty string.
This was done to disable "Pin" when no launcher URL is available but not properly executed in "Start New Instance"

Test Plan:
5.12 branch

• "Launch new instance" is now disabled for e.g. wine notepad where it wouldn't work anyway (second change)
• "Pin" is still properly checked depending on whether the task is pinned or not (first change)

Reviewers: Plasma, hein

Reviewed By: Plasma, hein

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D13069