DONTDROP haskellPackages.mkDerivation: Fix `cannot coerce null to a s…

…tring`

I think `glibcLocales` in `all-packages.nix` should use `targetPlatform`
instead of `hostPlatform` in the conditionals but that gives infinite
recurision in a repl with the options below

Fixes

```
$ nix repl --file . --system aarch64-darwin
nix-repl> pkgsCross.aarch64-multiplatform.haskellPackages.ghc-lib.env
error:
       … while evaluating attribute 'LOCALE_ARCHIVE' of derivation 'ghc-shell-for-ghc-lib-aarch64-unknown-linux-gnu-9.6.6.20240701-aarch64-unknown-linux-gnu'
       error: cannot coerce null to a string: null
```

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

@@ -802,7 +802,7 @@ stdenv.mkDerivation ({
         buildInputs =
           otherBuildInputsSystem;
         LANG = "en_US.UTF-8";
-        LOCALE_ARCHIVE = lib.optionalString (stdenv.hostPlatform.libc == "glibc") "${buildPackages.glibcLocales}/lib/locale/locale-archive";
+        LOCALE_ARCHIVE = lib.optionalString (stdenv.hostPlatform.libc == "glibc" && buildPackages.glibcLocales != null) "${buildPackages.glibcLocales}/lib/locale/locale-archive";
         "NIX_${ghcCommandCaps}" = "${ghcEnv}/bin/${ghcCommand}";
         "NIX_${ghcCommandCaps}PKG" = "${ghcEnv}/bin/${ghcCommand}-pkg";
         # TODO: is this still valid?