applets/digital-clock: ensure CompactRep item exists before querying it
requested to merge ngraham/plasma-workspace:fix-tm-compactrep-mousearea-containsmouse-error into master
Fixes main.qml:111: TypeError: Cannot read property 'containsMouse' of null error.
The error is harmless because the feature it supports works anyway because the CompactRepresentation item is created quickly, but it still pollutes logs.