Returning NULL?

Is there some way to return something like a NULL or undefined from a Bro script function? I'm looking to set a field in a logged record via a function call but would ideally like to be able to return an unset value from that function if needed.

There is no way to direcly return null. The two "workarounds" that come to
mind at the moment are to either return a record with one optional field -
which you can leave unset to represent null. Or return a vector where you
either can add the element when present, or let it be zero-size when 0.