migrations: Fix system bot realm not having a name.

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).

zerver/lib/server_initialization.py

@@ -13,7 +13,7 @@ def server_initialized() -> bool:
 def create_internal_realm() -> None:
     from zerver.lib.actions import do_change_can_forge_sender
 
-    realm = Realm.objects.create(string_id=settings.SYSTEM_BOT_REALM)
+    realm = Realm.objects.create(string_id=settings.SYSTEM_BOT_REALM, name="System bot realm")
     RealmAuditLog.objects.create(
         realm=realm, event_type=RealmAuditLog.REALM_CREATED, event_time=realm.date_created
     )

zerver/migrations/0345_alter_realm_name.py

@@ -1,15 +1,32 @@
 # Generated by Django 3.2.6 on 2021-09-08 22:35
 
+from django.conf import settings
 from django.db import migrations, models
+from django.db.backends.postgresql.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+
+def rename_system_bot_realm(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    # Prior to this migration, the system bot realm had name incorrectly set to null.
+    Realm = apps.get_model("zerver", "Realm")
+    Realm.objects.filter(name=None, string_id=settings.SYSTEM_BOT_REALM).update(
+        name="System bot realm"
+    )
 
 
 class Migration(migrations.Migration):
+    atomic = False
 
     dependencies = [
         ("zerver", "0344_alter_emojiset_default_value"),
     ]
 
     operations = [
+        migrations.RunPython(
+            rename_system_bot_realm,
+            reverse_code=migrations.RunPython.noop,
+            elidable=True,
+        ),
         migrations.AlterField(
             model_name="realm",
             name="name",