conversationrequest: avoid to accept with a non-existing conversation

We recently changed getContactDetails() to get the removed state when
needed. However, getOneToOneConversation was now returning a removed
conversation causing the request to be ignored if both sides removed
them previously.

GitLab: #1619
Change-Id: Ie7464460609d6c1b6b3774318fb50116b7408f0a