Minor fixes on VTS tests

- call startSvStatus()/startNmea() before calling start(). This will
  avoid missing the first SvStatus
- in TestGnssSvInfoFieldsTest, avoid calling list::back() if the list is
  empty.

Bug: 235062809
Test: atest VtsHalGnssTargetTest
Change-Id: I612d42e23085dbb0b32da328869bfd443ee66362