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.

pkgs/development/haskell-modules/hoogle.nix

@@ -63,10 +63,13 @@ buildPackages.stdenv.mkDerivation {
   passAsFile = ["buildCommand"];
 
   buildCommand = ''
-    ${lib.optionalString (packages != [] -> docPackages == [])
+    ${let # Filter out nulls here to work around https://github.com/NixOS/nixpkgs/issues/82245
+          # If we don't then grabbing `p.name` here will fail.
+          packages' = lib.filter (p: p != null) packages;
+      in lib.optionalString (packages' != [] -> docPackages == [])
        ("echo WARNING: localHoogle package list empty, even though"
        + " the following were specified: "
-       + lib.concatMapStringsSep ", " (p: p.name) packages)}
+       + lib.concatMapStringsSep ", " (p: p.name) packages')}
     mkdir -p $out/share/doc/hoogle
 
     echo importing builtin packages