relayer-cli: proof generation returns empty array in case of an error

In relayer-cli's `proof.ts`, the [`getProofAtCount`](https://github.com/kleros/vea/blob/0adb33e07a0dc1c9d8265adcc332bf5ef273e569/relayer-cli/src/utils/proof.ts#L27) returns an empty array in case of an error in the subgraph query which is misleading and doesn't make sense.