when not enough headers, continue safely instead of aborting

closes https://github.com/kdeldycke/maildir-deduplicate/issues/16
The problem with this is that it's not a fine-grained approach, treating all exceptions in the same way.  This could lead to other bugs being accidentally swept under the carpet.  So it would be better to have explicit checks for missing headers within `compute_hash_key()`, and if `compute_hash_key()` decides it's incapable of computing a reliable hash key for whatever reason (e.g. insufficient headers) then it should return `(None, None)` and this code would handle that outcome in the same way as your `except` clause.

I hope that makes sense?  If not, don't hesitate to ask for clarification.  If so, would you be able to amend this PR accordingly via `git commit --amend` and update the PR via `git push -f`?  Thanks!