`parsePacket` returns `undefined` when the run's payload data is empty,
which crashed the test page on `!packet.timezone` (Sentry
TRIGGER-CLOUD-1AG: Cannot read properties of undefined (reading
'timezone')). Guard the timezone defaulting on `packet` being defined;
`safeParse(undefined)` then fails cleanly and the run is filtered out by
the existing `payload.success` check.
Adds a unit test covering the empty-payload regression case, the
timezone defaulting, and the no-crash on malformed payloads.
Dan Sutton
ced242086b