singlefileresourceconfigwidget.h - saving a empty path is a failure condition

Attempting to save an empty path is a failure condition. This can happen in the ical (single file) resource when the Ok/Apply buttons are pressed and the urlrequester is empty.

(Ideally, the Ok and Apply buttons would be disabled in the empty urlrequester case, but the logic for that is entwined in many different scenarios and resources.)

With this patch, if the user presses Ok/Apply with empty url the dialog pops-down harmlessly.