batch.every() is vacuously true on an empty array, so a POST with body []
was fulfilled with 200 [] instead of being reported as an unstubbed
request. The one mechanism whose job is to fail the suite on unrecognised
outbound traffic let it through. Reject an empty batch explicitly; no real
JSON-RPC batch is empty, so nothing legitimate is caught by it.
Demonstrated by execution with a throwaway probe firing four POSTs the
harness does not stub. Before: [] fulfilled and its probe passed, while
the bodyless, scalar-JSON and [null] probes were reported and failed.
After: all four reported and failed, with tests 1-27 still green.
Also corrects the comment above the guard. postData() is
buffer.toString("utf-8") || null in playwright-core, so a binary body is
decoded lossily into invalid JSON and reported by the catch, not returned
as null; only an absent or empty body yields null and reaches the type
guard. Both paths report, but the stated reason was wrong.
