perfetto_cmd: do not start incident reports for empty traces
perfetto_cmd: do not start incident reports for empty traces

Backport of aosp/1095032. Fixes the problem where when using "tracing
into an incident", empty traces would unintentionally cause an incident
to be created (either empty, or containing the last saved trace of this
kind).

This is preventing the use of incidentd->BetterBug upload path for
perfetto traces from droidfooders (as there are tracing configurations
which intentionally end up with empty traces, which should be discarded
without side-effects).

See b/138780197 for a longer explanation.

Test: manual on crosshatch-userdebug
Bug: 138649485
Merged-In: Ibed6b03f708a994d2c72788b5c251ded152679bc
Change-Id: Id75c451a34f0e04f2a7febf8bd162a89fc8699ce

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto