Fix broken code in PhabricatorMetaMTAMail::flattenHeaders()
Summary: Looping over the elements of an empty array while ignoring the parameter passed makes no sense. Looks like a typo.
Test Plan: Run static code analysis: `/src/applications/metamta/storage/PhabricatorMetaMTAMail.php:1104 Empty array passed to foreach`
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26416