overviewControls: Don't update work area box when headless

When headless, we don't have a monitor we can derive a size from, so don't even try, just let it be updated when we actually have a monitor that will show stuff.

Closes: #5831 (closed)

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Tested by @pnowack. Should be considered for code freeze exception, since it unbreaks launching shell before monitors are connected, or when connecting to a headless shell instance remotely.