[KStatusNotifierItem] Allow left click when menu is null

Summary:
If associatedWidget and menu are the same then instead of "activate"
action context menu is used. When both are null coparition is always
true but context menu can't be shown, since it is null.
It is partial solution for a problem described in https://bugs.kde.org/show_bug.cgi?id=365105.

CCBUG: 365105

Test Plan:
As described in the bug:
* create QSystemTrayIcon
* assign QMenu
* show, hide and show again
Both right click and left click do nothing.

After change left click works

Reviewers: davidedmundson, broulik, nicolasfella, #frameworks

Reviewed By: davidedmundson

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D24667