`eth_getBlockByNumber` returns empty response on Ethereum mainnet

Using the latest `dshackle` release `0.15` with the following minimal reproduction config:

```yaml
version: v1
port: 2449

proxy:
host: 0.0.0.0
port: 8545
routes:
- id: ethereum
blockchain: ethereum

cluster:
upstreams:
- id: quicknode-mainnet
chain: ethereum
disable-validation: true
connection:
ethereum:
rpc:
url: ""
ws:
url: ""
```

Despite upstreams providing responses for `eth_getBlockByNumber` (I verified by dumping upstream responses in `WsConnectionImpl`) dshackle returns an error to downstream clients. As an example reproduction using `cast`:

```sh
$ cast rpc eth_getBlockByNumber 0x160d0c4 true
Error: server returned an error response: error code -32002: No response or no available upstream for eth_getBlockByNumber
```

Sent against a known-good upstream directly, that produces the expected response.