session.activate's persisted-transcript refresh reconciled unconditionally
against getLatestSessionMessages, so a transient empty REST page (e.g. a
backend respawn racing its own state.db read after a wake/reconnect) wiped
a transcript the activate response had just restored. Guard it the same way
the activate payload itself already is guarded a few lines above: an empty
authoritative page never overrides a non-empty cached transcript.
chelsealong
f0748b451c