AppletQuickItem: Better handle missing mainScript
SharedQmlEnginePrivate::beginExecute checks whether the source is empty, which is a very reasonable thing to warn about.
However in that case it won't create a component for it, which is problematic because AppletQuickItem will query the component for errors, causing a crash. The code guards against the component being null, but inconsistenly.
Instead always create a component. If the source is empty the component will hold an error and the UI will show it
SENTRY: PLASMA-WORKSPACE-14EM