migrations: Fix ‘continue’ logic error in 0037.

The intention was to continue the outer ‘for’ loop, not the inner one
(but Python doesn’t have labelled ‘continue’).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/migrations/0037_disallow_null_string_id.py

@@ -19,9 +19,10 @@ def set_string_id_using_domain(apps: StateApps, schema_editor: BaseDatabaseSchem
                 try:
                     realm.string_id = prefix + str(i)
                     realm.save(update_fields=["string_id"])
-                    continue
+                    break
                 except IntegrityError:
                     pass
+            else:
                 raise RuntimeError(f"Unable to find a good string_id for realm {realm}")