Fix the error and make the argument nonnull

When trying to add nullability annotations to arguments in aosp/2381752,
conflicts come up.
See detail in https://android-build.googleplex.com/builds/pending/P47378145/aosp_bramble-userdebug/latest/view/logs/build_error.log

Bug: b/245972273
Test: atest mtp_ffs_handle_test
Change-Id: I841c57a404dbd0ee143257fe9106e3bcecbc8b3d