flatpak-dir: Set subpaths when calling Deploy() if not set

This adds `NULL` protection to the `subpaths` argument to
`flatpak_dir_system_helper_call_deploy()`, just like already exists for
other arguments.

It’s a second layer of defence against bugs like fixed in the previous
commit.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

common/flatpak-dir.c

@@ -2078,6 +2078,8 @@ flatpak_dir_system_helper_call_deploy (FlatpakDir         *self,
 {
   const char *empty[] = { NULL };
 
+  if (arg_subpaths == NULL)
+    arg_subpaths = empty;
   if (arg_previous_ids == NULL)
     arg_previous_ids = empty;