filter out bad convos in conversations_json
filter out bad convos in conversations_json

flag=none
fixes VICE-2926

the sentry error described in the ticket
were trying to grab Conversation info
from ConversationParticipant records that do
not have a Conversation - which led to the sentry
errors

believing that all ConversationParticipants have
Conversations, then let's just not process
info for these bad convos

test plan:
- existing specs must pass
- with :react_inbox OFF
- verify that you can see conversations
in the old inbox page

Change-Id: I7c0ecaf9011826aab62507573f555007866429fc
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/297293
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Jason Gillett <12d37bea14b8257b31fd2f970455158adbd46adb@instructure.com>
Product-Review: Jason Gillett <12d37bea14b8257b31fd2f970455158adbd46adb@instructure.com>
QA-Review: Omar Soto-Fortuño <2ca100155bc0669318a7e6f3850629c7667465e1@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms