haskell: Fix error when languageServer is set to null

[sestrella]

The languageServer option type inside the haskell language module is set to lib.types.nullOr lib.types.package, however, the packages section does not check for null values causing the following error:

error: A definition for option `perSystem.aarch64-darwin.devenv.shells.default.packages."[definition 1-entry 2]"' is not of type `package'. Definition values:
       - In `/nix/store/95czchz5fgjcak3y1y5cizx22irqa09w-source/src/modules/languages/haskell.nix': null

Relevant code snippet:

languages.haskell = {
  enable = true;
  languageServer = null;
};