[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