check-pol calls dn42-schema with invalid args if filter.txt has been edited

There's no need to validate $LINE[1] if $LINE[2] is nonempty. (Except if
git emits a path with "//", but I don't think this can happen.)

check-pol

@@ -12,7 +12,7 @@ BASE="$(dirname "$BASE")"
 cd "$BASE" || exit 1
 
 git diff --name-only $1 | while IFS='/' read -ra LINE; do
-if [[ "${LINE[0]}" = "data" ]]; then
+if [[ "${LINE[0]}" = "data" && -n "${LINE[2]}" ]]; then
 	utils/schema-check/dn42-schema.py -v policy ${LINE[1]} ${LINE[2]} $2
 fi
 done