The streaming response translator's same-format fast path returned
`[chunk]` unconditionally, so the end-of-stream null/flush signal
(chunk === null) propagated as a literal `[null]`. Downstream this surfaced
as an empty `data: null` SSE event between chunks and crashed strict clients
(e.g. Factory Droid BYOK on /v1/responses). The fast path now returns `[]`
for the null flush while still passing real chunks through unchanged.
Reported-by: thaitryhand (https://github.com/decolua/9router/issues/1052)
Co-authored-by: thaitryhand <248103256+thaitryhand@users.noreply.github.com>
