haskell/hoogle.nix: make an empty packages list possible

Building hoogle environments with an empty list of packages threw a built-time
error. This was particularly confusing when giving e.g. `(hps: [hps.Cabal])`
which, since `Cabal` is set to `null` (because it’s a distribution package),
would have lead to the same error as an empty list.

It was not useful to throw an error at all, because sometimes one only wants to
build an environment for the distribution packages; also the default value for
packages wasn’t even a valid value, so it is removed.