This prevented migration 0345
(517c2ed39d / #19696) from applying on
systems that were created after the refactoring that resulted in the
system bot realm potentially having null as its name.
(We've already confirmed that normal realms, created via
`do_create_realm`, shouldn't be able to have this unusual state).
This was likely initiall created with null=True in
5c5ffd6ea3 just because we didn't have a
plan for backfilling this field, but I verified that Zulip Cloud has
no realms without a name set, and that's the place most likely to have
any form of super-legacy nameless realms.
So we can clean up this aspect of the data model without a special
migration to do something with existing realms with name=None (which I
suspect would have resulted in a 500 anyway).
Anders Kaseorg
Zixuan James Li
Tim Abbott