Skip empty gametypes in level select menu

Useful if a total-conversion mod doesn't want to include support for a gametype

If all gametypes are empty, switching gametypes will lead into an infinite loop. I can think of two ways to fix that:

• The lazy way: If the while loop lands back at the gametype it started with, it just gives up and stays with that gametype.
• The proper way: If no multiplayer map exists at all, hide the multiplayer menu entirely (since there's nothing useful you can do in it anyway).

After some dev discussion, the proper way seems to be contentious, so for now it should probably be fixed the lazy way.

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• 725826a6 - Avoid infinite loops if no maps are selectable (y tho)

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• 78ec2108 - hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

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resolved all discussions

merged

mentioned in commit fa9c2f43

mentioned in merge request !868 (merged)