Building hoggle database fails if build depends have no packages with documentation

[pjones]

Strange error until you add enough packages to the build-depends:

error: while evaluating the attribute 'NIX_GHC' of the derivation 'ghc-shell-for-inhibit-screensaver-0.0.0.0-0' at /nix/store/j2941sr319c4mighd0r2b95qq2gi3ykh-source/pkgs/development/haskell-modules/generic-builder.nix:613:16:
while evaluating the attribute 'paths' of the derivation 'ghc-8.8.4-with-packages' at /nix/store/j2941sr319c4mighd0r2b95qq2gi3ykh-source/pkgs/build-support/trivial-builders.nix:7:7:
while evaluating the attribute 'buildCommand' of the derivation 'hoogle-local-0.1' at /nix/store/j2941sr319c4mighd0r2b95qq2gi3ykh-source/pkgs/development/haskell-modules/hoogle.nix:58:3:
while evaluating 'optionalString' at /nix/store/j2941sr319c4mighd0r2b95qq2gi3ykh-source/lib/strings.nix:180:5, called from /nix/store/j2941sr319c4mighd0r2b95qq2gi3ykh-source/pkgs/development/haskell-modules/hoogle.nix:66:7:
while evaluating 'concatMapStringsSep' at /nix/store/j2941sr319c4mighd0r2b95qq2gi3ykh-source/lib/strings.nix:88:5, called from /nix/store/j2941sr319c4mighd0r2b95qq2gi3ykh-source/pkgs/development/haskell-modules/hoogle.nix:69:10:
while evaluating anonymous function at /nix/store/j2941sr319c4mighd0r2b95qq2gi3ykh-source/pkgs/development/haskell-modules/hoogle.nix:69:40, called from undefined position:
value is null while a set was expected, at /nix/store/j2941sr319c4mighd0r2b95qq2gi3ykh-source/pkgs/development/haskell-modules/hoogle.nix:69:43

[pjones]

Upstream issue: NixOS/nixpkgs#82245