fix strlen in PhabricatorMailManagementShowOutboundWorkflow

Summary: Ref T15064

Test Plan: run ./bin/mail show-outbound --id 1 on a non-html email with php 8.1, no crash.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Tags: #php_8_support

Maniphest Tasks: T15064

Differential Revision: https://we.phorge.it/D25275