runfix: 1:1 conversation with user without key packages

[PatrykBuniX]

Description

Handles a 1:1 conversation with a user that don't have any key packages available to establish a MLS 1:1 conversation.

Screenshots/Screencast (for UI changes)

Checklist

• [x] PR has been self reviewed by the author;
• [x] Hard-to-understand areas of the code have been commented;
• [x] If it is a core feature, unit tests have been added;