Drop DEFAULT constraint post secrets migration

ideally we do this here
```suggestion
ADD COLUMN IF NOT EXISTS encryption_key TEXT NOT NULL DEFAULT '',
```

and then later when migrattions are done, this can come in
```sql
ALTER TABLE superposition.workspaces ALTER COLUMN encryption_key DROP DEFAULT;
```

_Originally posted by @ayushjain17 in https://github.com/juspay/superposition/pull/773#discussion_r2736858536_