fix: handle empty CSV file without crashing (#3196)

* fix: handle empty CSV file without crashing

Fixes #3195

Accessing self.csv_data[0] on an empty CSV raised an IndexError.
Moved the column uniformity check inside the existing if/else on
csv_data so empty files log a warning and return an empty document.

Signed-off-by: Eugenio-BAYE <baye.eugenio.egnb@gmail.com>

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Peter W. J. Staar <91719829+PeterStaar-IBM@users.noreply.github.com>

---------

Signed-off-by: Eugenio-BAYE <baye.eugenio.egnb@gmail.com>
Signed-off-by: Peter W. J. Staar <91719829+PeterStaar-IBM@users.noreply.github.com>
Co-authored-by: Peter W. J. Staar <91719829+PeterStaar-IBM@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>