Storage Policy Editor Silently Fails To Save Cleared USING/WITH CHECK Expressions

In createPayloadForUpdatePolicy, when a field is cleared its value becomes undefined; during JSON serialization, undefined keys are dropped, so the API never receives the update. The UI validation allows clearing, but no conversion to null is performed.

1. Go to Storage → Policies, create an UPDATE policy with both USING and WITH CHECK expressions. 2. Edit the policy, clear the WITH CHECK expression, click Review. 3. Notice the preview shows invalid SQL like `ALTER POLICY ... WITH CHECK ();`. 4. Click Save; a success toast appears. 5. Reopen the policy or query pg_policies to see the expression is still present.

This fix assumes the backend API accepts null to drop the expression. If the backend expects an empty string or a different sentinel, clearing may still fail. Additionally, the review modal may still preview invalid SQL (e.g., `WITH CHECK ()`), so a separate fix in the preview generator is required. If the API cannot handle null, rollback by reverting this change and instead preventing clearing in the UI.