ArrowCursor reads NULL time and time zone values as empty strings #1218

Part of #1212.

Summary

ArrowCursor reads NULL time, time with time zone, and timestamp with time zone values as empty strings.

Facts

Measured on master b2d7aa2 (pyarrow 26.0.0), for S3 result files and managed results: an all-NULL column of these types is string with the value '', while the fetch methods return None.

These columns are text in ArrowCursor, so they inherit the reader's handling of strings. docs/null_handling.md documents that ArrowCursor cannot tell NULL from '' for strings. For these types, Athena never writes an empty string as a value, so '' can only be NULL.

Expected behavior

These columns hold null for SQL NULL, following #1212. Whether varchar changes too is outside this issue.