Patch 03

[SteelBlueZK]

Potential fix for #5801

This essentially silent fails, "If this Unitdef is not a unitdef, then it is not a commander"

[sprunk]

Personally I think the error is good (since the mod is passing an incorrect value and should just stop doing that) so perhaps the ideal improvement would have been to make it more helpful:

if not ud then
  error("Spring.Utilities.isComm() called with invalid arg: " .. tostring(ud))
end

But as-is the PR is consistent with how everything is polluted with a billion nil checks.