handle multipart incoming email with no html part

fixes CNVS-15232

test plan
- reply to a conversation message notification email with a
 multipart message that doesn't have an html part
- ensure that the reply shows up in canvas
- ensure that no error report is generated from the incoming
 message processor

Change-Id: If7b4913fe28815312360e58e50f47bf44ff2ead2
Reviewed-on: https://gerrit.instructure.com/40971
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>