Studio: policy editor can emit an empty `with check ()` when creating a non-INSERT policy

## Bug report Creating an RLS policy for a non-INSERT command from the Policies editor can emit a `with check ( )` clause with empty parens, which Postgres rejects. Hit while creating a policy on `realtime.messages` from the built-in template `Allow listening for broadcasts for authenticated users only`. ### Payload sent `POST /api/platform/pg-meta/default/query` → HTTP 400: ```sql create policy "Allow listening for broadcasts for authenticated users only" on realtime.messages as permissive for select to authenticated using ( realtime.messages.extension = 'broadcast') with check ( ); ``` ### Error ``` ERROR: syntax error at or near ")" LINE 1: ...( realtime.messages.extension = 'broadcast') with check ( ); ^ ``` Dropping the trailing `with check ( )` makes the statement succeed. Not version-specific — empty parens are a parse error on any Postgres, and `WITH CHECK` isn't valid for SELECT/DELETE in the first place. ### Environment Self-hosted Studio built from master.