monitor: Allow %NULL mode in calculate_mode_scale()

Closed Florian Müllner requested to merge wip/fmuellner/fix-headless-segfault into master

If no global scaling factor has been set, we currently segfault when called with a %NULL mode parameter, which happens when calculating the scale for the current mode while no modes are available.

Adding %NULL checks in the corresponding callers would mean that we'd have to duplicate the global scaling factor handling, so instead consider %NULL mode parameters valid and handle them gracefully.

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