haskellPackages.shellFor: Work around hoogle generation failure
haskellPackages.shellFor: Work around hoogle generation failure

Workaround for https://github.com/NixOS/nixpkgs/issues/82245

Although this doesn't tackle the root cause of a null package sneaking
in (via executableHaskellDepends), it does effectively treat the symptom
by just ignoring any null packages.

Seeing as that issue has been open for more than a year I think this
band-aid is necessary.

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