systemd.disallowedReferences: avoid null values

hydra-queue-runner complains like this:
  loading build 302783324 (nixpkgs:cross-trunk:rpi-musl.mpg123.x86_64-linux)
  queue monitor: error:
         … while loading build 302783324:
         … while parsing derivation '/nix/store/8r46dmzzchv9jhb09y8dlih4h5k6smkm-systemd-minimal-armv6l-unknown-linux-musleabihf-257.6.drv'
         error: attribute 'disallowedReferences' must be a list of strings
  checking the queue for builds...
  loading build 302783295 (nixpkgs:cross-trunk:rpi-musl.mpg123.aarch64-darwin)

pkgs/os-specific/linux/systemd/default.nix

@@ -883,7 +883,11 @@ stdenv.mkDerivation (finalAttrs: {
   disallowedReferences =
     lib.optionals (stdenv.buildPlatform != stdenv.hostPlatform)
       # 'or p' is for manually specified buildPackages as they dont have __spliced
-      (builtins.map (p: p.__spliced.buildHost or p) finalAttrs.nativeBuildInputs);
+      (
+        builtins.map (p: p.__spliced.buildHost or p) (
+          builtins.filter (p: p != null) finalAttrs.nativeBuildInputs
+        )
+      );
 
   passthru = {
     # The `interfaceVersion` attribute below points out the incompatibilities