Session + presentation fields no longer overwritten when columns are absent
from the CSV (start_datetime, end_datetime, sort, code, name, abstract_code).
Presentation lookup no longer requires presentation_name to find an existing
record — only INSERT requires it. This fixes presenter email updates when
re-importing a partial CSV that has presenter_code but no presentation_name.
Presenter email is now only written when the presenter_email column is
present in the CSV, preventing accidental email clears on partial imports.
Presenter INSERT/UPDATE gate restored to original logic: requires
(presenter_code or presenter_number) and presenter_given_name.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
21e0825841