initialize_voyager_db: Do nothing if a realm already exists.

This should help avoid confusing error messages for anyone
accidentally running this twice.

In particular, this also makes it easier to run Zulip inside
Kubernetes, since one doesn't need to worry about duplicate calls.

zerver/management/commands/initialize_voyager_db.py

@@ -30,6 +30,9 @@ class Command(BaseCommand):
                             help='The number of extra users to create')
 
     def handle(self, *args: Any, **options: Any) -> None:
+        if Realm.objects.count() > 0:
+            print("Database already initialized; doing nothing.")
+            return
         realm = Realm.objects.create(string_id=settings.INTERNAL_BOT_DOMAIN.split('.')[0])
 
         names = [(settings.FEEDBACK_BOT_NAME, settings.FEEDBACK_BOT)]