hwdb-update: make sure it works when run from meson

let's make the argument optional again, so that the command line "ninja
-C build hwdb-update" runs works.

tools/meson-hwdb-update.sh

@@ -4,9 +4,11 @@ set -eu
 cd "$1"
 
 unset permissive
-if [ "$2" = "-p" ]; then
+if [ "${2:-}" = "-p" ]; then
         permissive=1
         shift
+else
+        permissive=0
 fi
 
 if [ "${2:-}" != "-n" ]; then (