[Scene] Use NULL instead of COND checks when appropriate

[AThousandShips]

Restricted to scene (it was enough work there, will expand in a later PR)

Tried to get everything but might have missed some

[KoBeWi]

Looks fine.
NULL is better than COND, because it provides better default message. It doesn't really matter when used with MSG.

[AThousandShips]

I agree, but I'd argue that when the context isn't obvious (though most cases here are, as they were the easiest to just search/replace) I find that the type of the check helps with readability of the code, when it's a member variable

[akien-mga]

Thanks!

[AThousandShips]

Thank you!