Don't return error when uid entry don't exsit

When query per uid stats, do not return error if that uid does not have
any uid stats entry in AppUidStatsMap. It might be that uid doesn't
generate any traffic at all.

Bug: 124764595
Test: android.net.cts.TrafficStatsTest with wifi and data off.
      libnetdbpf_test
Change-Id: Ibe986e2d6ab362868c6b7b63b059111af0b5706f