[lib] fix changing thread description on native for thick threads

Summary:
ENG-9316.

The description is a nullable field (link), so whenever clicking editing button for not edited or not set description here it causes changing value from undefined to undefined and a result nothing changes. We're not entering this branch which renders TextInput.

For thin threads, it works only because for some reason keyserver is replacing undefined description with an empty string here.

The alternative solution is to create a thick thread with an empty string as a description (instead of undefined), but I don't like this because types still allow for descryption to be nullable so this bug might be caused again in the future.

Test Plan:

1. Change thread settings on native works for thin and thick threads.
2. Notifs works.
3. Changing the description to the same value is a no-op.

Reviewers: tomek, ashoat

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13447