ghcWithHoogle fails with only null dependencies

[ElvishJerricco]

If you use ghcWithHoogle, passing only null dependencies, it will crash with

error: value is null while a set was expected, at /nix/store/v8s18h7r1fxmajdzn0f1ybpyl7m2c1di-nixos-17.09.2138.48bf1f43cd/nixos/pkgs/development/haskell-modules/hoogle.nix:73:43

Example failure:

{ pkgs ? import <nixpkgs> {} }:

pkgs.haskellPackages.ghcWithHoogle (p: with p; [base bytestring])

Example success:

{ pkgs ? import <nixpkgs> {} }:

pkgs.haskellPackages.ghcWithHoogle (p: with p; [base bytestring text])

[ElvishJerricco]

/cc @peti

[ElvishJerricco]

This comes up especially when using callCabal2nix on a project with only core dependencies like base and bytestring which are null in haskellPackages.