wrapGAppsHook: don't add empty variables

Adding empty variables can lead to this problem:

```diff
 wrapProgram \
     ./pye_menu_shell \
     --prefix PATH : /nix/store/4c3z5r6yxsf2cxwwyazhdn92xixn4j5b-python3-3.7.5/bin:/nix/store/b3l3niilvqcxcsbxmd6sgqk1dy1rk81c-pye-menu-1.0/bin:/nix/store/y8j1cfj8d9r5rbbxc22w7hnfjw5f4fd3-cairo-1.16.0-dev/bin:/nix/store/6mg7lfbdh9pgx7pbxr3544qqbrigdl1q-freetype-2.10.1-dev/bin:/nix/store/gpszqcy0xi0lavbbjdq82zkkjp3jbp2a-bzip2-1.0.6.0.1-bin/bin:/nix/store/031c5pk5lzabgmpqpyd46hzi625as6bp-libpng-apng-1.6.37-dev/bin:/nix/store/f8kl7kmpv130aw9zm542p74a3hg0yc13-fontconfig-2.12.6-bin/bin:/nix/store/bqp30vkncmm222mjvwggz0s7p318sflj-expat-2.2.7-dev/bin:/nix/store/w57xa8g4s4aviwmqwgra7m5hwj2b005m-glib-2.60.7-dev/bin:/nix/store/v5d4966ahvfir2hwpv003022f3pb7vik-gettext-0.19.8.1/bin:/nix/store/qpvxhl1jr0fxnrx9idnpdagqs00m5m2z-glib-2.60.7/bin \
     --set PYTHONNOUSERSITE true \
     --set GDK_PIXBUF_MODULE_FILE /nix/store/7ddlakx6xjczqbfs80xjd14f30fzadws-gdk-pixbuf-2.38.1/lib/gdk-pixbuf-2.0/2.10.0/loaders.cache \
     --prefix XDG_DATA_DIRS : /nix/store/0snjc1qg89zqn3v35l9d55xrykh9nj5c-gtk+3-3.24.10/share/gsettings-schemas/gtk+3-3.24.10:/nix/store/b41z51vdv11n6df8ki5vj8dynxw98f9l-gsettings-desktop-schemas-3.32.0/share/gsettings-schemas/gsettings-desktop-schemas-3.32.0:/nix/store/0snjc1qg89zqn3v35l9d55xrykh9nj5c-gtk+3-3.24.10/share/gsettings-schemas/gtk+3-3.24.10 \
-     --prefix GST_PLUGIN_SYSTEM_PATH_1_0 : \
+     --prefix GST_PLUGIN_SYSTEM_PATH_1_0 : "" \
     --prefix GI_TYPELIB_PATH : /nix/store/0snjc1qg89zqn3v35l9d55xrykh9nj5c-gtk+3-3.24.10/lib/girepository-1.0:/nix/store/z29l5xaaxh1s0697mcldj71ab0zshry1-librsvg-2.44.15/lib/girepository-1.0:/nix/store/pija1xzm7izxfb5m2hvhvlwp1l38ffxa-gobject-introspection-1.60.2/lib/girepository-1.0 \
-     --prefix GRL_PLUGIN_PATH :
+     --prefix GRL_PLUGIN_PATH : ""
```
Where the diff is to highlight the problem: we don't have a valid value
for GST_PLUGIN_SYSTEM_PATH_1_0 or GRL_PLUGIN_PATH, and instead of
passing the empy string, the empty string gets unquoted somewhere, so we
end up passing no arguments, thus the parser in wrapProgram takes
--prefix as the argument of GST_PLUGIN_SYSTEM_PATH_1_0, and then
GI_TYPELIB_PATH is missing it's --prefix so wrapProgram complains/dies.

The easiest change is to not add empty arguments to the wrapper

pkgs/build-support/setup-hooks/wrap-gapps-hook.sh

@@ -42,7 +42,9 @@ wrapGAppsHook() {
   fi
 
   for v in ${wrapPrefixVariables:-} GST_PLUGIN_SYSTEM_PATH_1_0 GI_TYPELIB_PATH GRL_PLUGIN_PATH; do
-    gappsWrapperArgs+=(--prefix "$v" : "${!v}")
+    if [ -n "${!v}" ]; then
+      gappsWrapperArgs+=(--prefix "$v" : "${!v}")
+    fi
   done
 
   if [[ -z "${dontWrapGApps:-}" ]]; then