Fix crash when starting game with null map

[11EJDE11]

This issue occurs when all players are marked as Ready, and the host quickly switches to a map that some players don't have before clicking Start. Players missing the map will crash.

The crash happens in GameLobbyBase.cs within WriteSpawnIni at the if (Map.IsCoop) check. This fix prevents the game reaching that line with a null map.

GameProcessExited is called to inform the host that the player has left the game. Without this, the host would still consider the player as IsInGame which would cause issues (I think) similar to #641.

The match statistics changes are because GameProcessExited calls them which won't work with a null map.
If it's preferred, instead of calling GameProcessExited and making changes to match statistics, we could just call the relevant lines of GameProcessExited directly.

            ClearReadyStatuses();
            CopyPlayerDataToUI();
            UpdateDiscordPresence(true);

Not sure which is best or preferred.

[github-actions]

Nightly build for this pull request:

• artifacts-TS.zip
• artifacts-Ares.zip
• artifacts-YR.zip
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[SadPencil]

Whenever this line is executed, does it always mean the game process is exited? If not, even if the implementation is the same, we should not call such a method. I suggest you made another method, something like HandleNullMap() or something