models: Make realm.name no longer nullable.

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).

zerver/models.py

@@ -207,7 +207,7 @@ class Realm(models.Model):
     id: int = models.AutoField(auto_created=True, primary_key=True, verbose_name="ID")
 
     # User-visible display name and description used on e.g. the organization homepage
-    name: Optional[str] = models.CharField(max_length=MAX_REALM_NAME_LENGTH, null=True)
+    name: str = models.CharField(max_length=MAX_REALM_NAME_LENGTH)
     description: str = models.TextField(default="")
 
     # A short, identifier-like name for the organization.  Used in subdomains;