[FIX] base_import: trying to import an empty file

Various bits would lead to odd tracebacks or a complete lack of
actionable feedback.

Discover that a binary field set to b'' will return None when read back,
handle that, then handle reading an empty CSV file, then handle the
resulting iterator having no lines whatsoever when trying to match
headers.

At least for CSVs, we're now properly telling users that their file
seems to have no content when they literally upload an empty file.

0 files changed, 0 insertions, 0 deletions