Studio: clearing a storage policy's WITH CHECK or USING expression is silently not saved

# Bug report - [x] I confirm this is a bug with Supabase, not with my own application. - [x] I confirm I have searched the [Docs](https://docs.supabase.com), GitHub [Discussions](https://github.com/supabase/supabase/discussions), and [Discord](https://discord.supabase.com). ## Describe the bug In the Storage policy editor, editing an existing policy and clearing one of its SQL expressions does not remove the expression. The dashboard shows a success toast, but the old expression is still on the policy. The cause is in `createPayloadForUpdatePolicy` in `apps/studio/components/interfaces/Storage/StoragePolicies/StoragePolicies.utils.ts`. When the user empties the field, the change is detected but the payload field is set to `undefined`. That key is dropped during JSON serialization, so the API never receives the change. This is reachable in the UI because validation in `PolicyEditorModal/index.tsx` allows it. For an `UPDATE` policy the `WITH CHECK` field can be cleared as long as `USING` is filled, and for an `ALL` policy both fields can be cleared. The review step also previews invalid SQL in this case, for example `ALTER POLICY "My policy" ON "storage"."objects" WITH CHECK ();`. ## To Reproduce 1. Go to Storage, then Policies, and create a policy for the `UPDATE` operation with both a `USING` and a `WITH CHECK` expression. 2. Edit the policy, clear the `WITH CHECK` expression, and click Review. 3. The preview shows `... WITH CHECK ();`, which is invalid SQL. 4. Click Save. A success toast appears. 5. Reopen the policy. The `WITH CHECK` expression is still there. You can also verify with `select * from pg_policies where schemaname = 'storage';`. ## Expected behavior Clearing an expression should either actually remove it from the policy, or the editor should not allow clearing the field if removal is not supported. It should not preview invalid SQL and then save nothing. ## Screenshots Not applicable, this is a logic issue in the payload construction. ## System infor…