rows_from_file() crashes with csv.Error on empty or whitespace-only files

[ikatyal2110]

When rows_from_file() is called with an empty file (or a file containing only whitespace) and no explicit format argument, it raises an uncaught csv.Error: Could not determine delimiter.

Repro:

import io
from sqlite_utils.utils import rows_from_file

rows, fmt = rows_from_file(io.BytesIO(b""))
list(rows)  # raises csv.Error: Could not determine delimiter

Root cause: In utils.py, the auto-detect branch strips the first 2048 bytes and passes the result to csv.Sniffer().sniff(). When first_bytes.strip() is empty (empty file or whitespace only), sniff("") raises csv.Error rather than returning gracefully.

The fix is to return an empty iterator before reaching sniff() when first_bytes is empty.