haskellPackages.mkDerivation: make license optional

`stdenv.mkDerivation` does not require meta.license to be passed, so there
is no reason `haskellPackages.mkDerivation` needs to enforce this. This
would free up cabal2nix to not report a license if it is not sure.

As I have argued in NixOS/cabal2nix#677 (comment),
it is better not to report a license than reporting an inaccurate one.

This would also allow to stop generating arbitrary strings as licenses
in cabal2nix to remove string values to facilitate more cleanliness in
the spirit of #445672, though the
question is whether it is wise to remove the meta data altogether.

Author: sternenseemann

pkgs/development/haskell-modules/generic-builder.nix

@@ -96,14 +96,14 @@ in
   libraryFrameworkDepends ? [ ],
   executableFrameworkDepends ? [ ],
   homepage ? "https://hackage.haskell.org/package/${pname}",
-  platforms ? with lib.platforms; all, # GHC can cross-compile
+  platforms ? lib.platforms.all, # GHC can cross-compile
   badPlatforms ? lib.platforms.none,
   hydraPlatforms ? null,
   hyperlinkSource ? true,
   isExecutable ? false,
   isLibrary ? !isExecutable,
   jailbreak ? false,
-  license,
+  license ? null,
   enableParallelBuilding ? true,
   maintainers ? null,
   teams ? null,
@@ -1031,10 +1031,11 @@ lib.fix (
       };
 
       meta = {
-        inherit homepage license platforms;
+        inherit homepage platforms;
       }
       // optionalAttrs (args ? broken) { inherit broken; }
       // optionalAttrs (args ? description) { inherit description; }
+      // optionalAttrs (args ? license) { inherit license; }
       // optionalAttrs (args ? maintainers) { inherit maintainers; }
       // optionalAttrs (args ? teams) { inherit teams; }
       // optionalAttrs (args ? hydraPlatforms) { inherit hydraPlatforms; }