NPE fix: log sensor timestamp only when sensor is not empty

I tried writing test for it but the problem is making `Trace.isTagEnabled` return true.
And anyway this is easy fix so hopefully test is just nice to have.

Test: manual
Fixes: 401728609
Flag: EXEMPT trivial fix
Change-Id: Ib4f8b962d05aff9eaaf23fc865bf53e30a4231ba