{wrapGAppsHook, patchShebangs}: fix issues with set -u (#366553)

pkgs/build-support/setup-hooks/patch-shebangs.sh

···
 
 patchShebangs() {
     local pathName
-    local update
+    local update=false
 
     while [[ $# -gt 0 ]]; do
         case "$1" in

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

···
     fi
 
     for v in ${wrapPrefixVariables:-} GST_PLUGIN_SYSTEM_PATH_1_0 GI_TYPELIB_PATH GRL_PLUGIN_PATH; do
-        if [ -n "${!v}" ]; then
+        if [ -n "${!v:-}" ]; then
             gappsWrapperArgs+=(--prefix "$v" : "${!v}")
         fi
     done