don't send "Some Name <>" reply-to header for SMS

omit the reply-to header if IncomingMail::ReplyToAddress is nil

test plan: outbound SMS messages should work

fixes CNVS-14745

Change-Id: I6ce2b1f453db0ddc5aed44a9cf8e1b4734d554b4
Reviewed-on: https://gerrit.instructure.com/39174
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>