winetricks: skip null dependencies

This can be used to reduce closure size when a wrapper uses a custom
WINE at runtime, or a different wine in PATH.

See pkgs/tools/package-management/protontricks/default.nix

pkgs/misc/emulators/wine/winetricks.nix

@@ -10,7 +10,8 @@ stdenv.mkDerivation rec {
 
   # coreutils is for sha1sum
   pathAdd = stdenv.lib.concatMapStringsSep ":" (x: x + "/bin")
-    [ wine perl which coreutils zenity curl cabextract unzip p7zip gnused gnugrep bash ];
+    (stdenv.lib.filter (x: x != null)
+      [ wine perl which coreutils zenity curl cabextract unzip p7zip gnused gnugrep bash ]);
 
   makeFlags = [ "PREFIX=$(out)" ];