Bun.write: deliver the whole payload to a FIFO instead of a torn prefix - #36025

This handoff fires before the empty-payload check, so `Bun.write(fifo_path, "")` — which previously resolved with `0` (open, skip write loop, discard the `ftruncate` EINVAL, close) — now routes to the async path, where an empty source has `store=None` and falls into `write_file_with_empty_source_to_destination` → `libc::truncate(fifo_path, 0)`, which returns `EINVAL` on Linux for any non-regular file and rejects the promise. Gate the handoff on `!str.is_empty()` here (and `!bytes.is_empty()` at