[applets/systemtray] Default value for ItemLoader.source

Summary: When none of the valid conditions are met, function should return default value for ItemLoader source. If not, error is printed in logs.

Test Plan: No warning about undefined value for url

Reviewers: #plasma_workspaces, #plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28259