Fix compilation on GHC 9

[josephcsible]

Fixes spell-music/csound-expression-dynamic#2.

[josephcsible]

As an alternative to moving where isEmptyExp is defined, you could instead leave it where it is, but switch it to use pattern matching, like this:

isEmptyExp :: E -> Bool
isEmptyExp (Fix (RatedExp{ ratedExpDepends = Nothing, ratedExpExp = EmptyExp })) = True
isEmptyExp _ = False

[anton-k]

Thanks for your help!