workspace: Fix an incorrect signal check

While commit 3094f863 was intended to cancel the ongoing idle hide timeout before we start a new one, a mistake slipped in there while rebasing: Obviously we should check if the signal id is NOT 0 here.

This didn't prevent timeouts being started while old ones are still running and did override this._idleHideOverlayId, which caused the old timeouts to run indefinitely after an overlay actor was destroyed because we fail early (and don't return TRUE) in _idleHideOverlay().