migrations: Change existing "(no topic)" to "general chat".

This adds a migration to update existing "(no topic)" objects
to "general chat".

Fixes zulip#23291.

Author: sofbe

zerver/migrations/0422_rename_no_topic_to_general_chat.py

@@ -0,0 +1,28 @@
+# Generated by Django 4.1.2 on 2022-11-01 12:55
+
+from django.db import migrations
+from django.db.backends.postgresql.schema import BaseDatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+
+def rename_no_topic_to_general_chat(
+    apps: StateApps, schema_editor: BaseDatabaseSchemaEditor
+) -> None:
+    Message = apps.get_model("zerver", "Message")
+    Message.objects.filter(subject="(no topic)").update(subject="general chat")
+
+
+class Migration(migrations.Migration):
+    atomic = False
+
+    dependencies = [
+        ("zerver", "0421_migrate_pronouns_custom_profile_fields"),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            rename_no_topic_to_general_chat,
+            reverse_code=migrations.RunPython.noop,
+            elidable=True,
+        ),
+    ]