fix(Mailer): Fix sendmail binary fallback

[joshtrichards]

• Resolves [Bug]: nextcloud 29.0.3 mail_smtpmode sendmail doesn't fork /usr/sbin/sendmail #46457

Summary

The non-deprecated BinaryFinder returns false not null when the fallback is needed (versus the deprecated one in OC_Helper):

server/lib/private/BinaryFinder.php

Line 45 in 465ad2f

public function findBinaryPath(string $program) {

Also add debug logging of which binary is found and fix test.

Some historical context:

• Move findBinaryFinder and isFunctionEnabled away from OC_Helper #33593

TODO

• Maybe: update tests to catch this sort of thing

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[joshtrichards]

/backport to stable29

[joshtrichards]

/backport to stable28