Fix NEWT MultiMonitor Regression: Use case w/o attached monitor, and NPE (Cache) in ScreenImpl.getVirtualMonitorDevice(..).

W/ RandR13, we can have a case of an Output/CRT w/o valid modes,
indicating a switched off or detached monitor.

ScreenImpl.getVirtualMonitorDevice(..) requires a Cache .. due to MonitorModeProps.streamInMonitorDevice(..)