Fixes apache#992.

Empty `pyarrow` arrays are considered falsy, which caused a `ResolveError` for required fields during scan operations.

This covers the issue reported in apache#992 where empty scan queries yielded a `ResolveError`. Specifically, this occurred under the following conditions:
- a table with an ordered, non-nullable string column
- a scan filtering for a non-existing value _within_ the range of the values in that particular column