Better Airdrop Production

This change allows the Production class and its children to take in multiple actors to be produced.
This is needed for #7055 so that the units produced can exit in an orderly fashion.
There's honestly really no need. The old implementation (the one you reverted to) was the equivalent of `return null;` , implemented like that because it was never needed until you tried to use it. I remember I checked thoroughly when we discussed that potential change.