[lib] Support creating private thread from user profile

Summary:
Before this diff, when no private thread existed, we wouldn't show a chat link in the user's own user profile. We didn't need to consider such a scenario since we always created a GENESIS_PRIVATE thread on account creation.

This diff changes behavior such that if !oldestPrivateThreadInfo (no private chat exists), we no longer return null from useUserProfileThreadInfo, and instead continue on to the createPendingPersonalOrPrivateThread call.

Depends on D13538

Test Plan: In combination with the rest of the stack, test creating a PRIVATE thread via thread list search on both web and native

Reviewers: tomek, kamil, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13539