Followup to ec5baaba: the previous commit dropped the unset/empty
telemetryId fallback. Restoring it preserves behavioral parity with
pre-PR code in case telemetryId is ever externally mutated to a
falsy value (the type permits "" even if normal flow never sets it).
Stop overwriting a stable vector-store UUID with the shared SDK
user_id. Existing OSS users were having their telemetry identity
silently swapped on next init, orphaning every prior PostHog event
tagged with the old id (the same regression class as the bug PR
#5040 set out to fix).
The shared SDK id is now adopted only when the vector store reports
"anonymous" / "anonymous-supabase" (fresh-install signals).
Mgeeeek