Fix #1364 - Avoid creating link elements in case of invalid value

In case of invalid value passed to _getLinkedCSS we should skip
creating link element to avoid unnecessary browser request.
Merge branch '1364_frame_setLinkedCSS' into dev-master
frame: Add unit test for issue #1364
frame: Build - fixes #1364
Merge branch 'dev-master' into dev-3.x