Works fine for me IF by 'Null' you mean the string 'Null'. So I'm betting you meant the field is actually null. Then perhaps this:
if(len(subfield(Department,'-',1)),subfield(Department,'-',1),'N/A') as Fund
Is there a string version of the alt() function? That would be simpler if so, but I'm not aware of one and don't see anything promising in the list of string functions. Maybe there's some other way to simplify it, though.
Edit: I was too slow with my post, and indeed my expression was more complicated than it needed to be. You can slightly simplify what Ducati 888 SP5 wrote, though, since 0 is false:
if(len(Department),subfield(Department,'-',1),'N/A') as Fund
Though I suppose that if the value of Department was '- Something', you might need my more-complicated version. I'm guessing you don't have data like that, though.