`register_record_batches` read the schema via unchecked `partitions.0[0][0]`,
which panics when a partition contains no record batches (e.g. the batches of
an empty pyarrow table, whose `to_batches()` returns `[]`).

Take the schema from the first available batch across all partitions and return
a clear error when there is none, mirroring the empty-table handling added for
`from_arrow_table` in apache#613. A non-empty later partition now also works instead
of panicking on an empty leading one.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix: no panic on empty partition in register_record_batches

`register_record_batches` read the schema via unchecked `partitions.0[0][0]`,
which panics when a partition contains no record batches (e.g. the batches of
an empty pyarrow table, whose `to_batches()` returns `[]`).

Take the schema from the first available batch across all partitions and return
a clear error when there is none, mirroring the empty-table handling added for
`from_arrow_table` in #613. A non-empty later partition now also works instead
of panicking on an empty leading one.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* refactor: address review feedback

- Use find_map(|p| p.first()) instead of flatten().next() to make the
  "first batch in any partition" intent clearer (per @kosiew).
- Extend regression test to also assert the empty outer partition list
  case ([]) raises the same ValueError.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>