graphql: getBlock return error for notfound hash, but null for notfound number

#### Expected behaviour

Whether the block-hash or block-number should always return an error or null, should be in the same behavior.

#### Actual behaviour

##### not found hash

> request

```graphql
{block(hash: "0xc1d1224983a2b43f2ea1b847337eda55e316e94603386e346abe3f6e08121028") { number }}
```

> response

```json
{
"errors": [
{
"message": "header for hash not found",
"path": [
"block"
]
}
],
"data": {
"block": null
}
}
```

##### not found number

> request

```graphql
{block(number: "0x100") { number }}
```

> response

```json
{
"data": {
"block": null
}
}
```

but the similar request in JSON-RPC all return `null`

> hash not found request

```json
{
"jsonrpc": "2.0",
"method": "eth_getBlockByHash",
"id": 1,
"params": [
"0xc1d1224983a2b43f2ea1b847337eda55e316e94603386e346abe3f6e08121028",
false
]
}
```

> number not found request

```json
{
"jsonrpc": "2.0",
"method": "eth_getBlockByNumber",
"id": 1,
"params": [
"0x100",
false
]
}
```

> the response

```json
{
"id": 1,
"jsonrpc": "2.0",
"result": null
}
```

IMO, we should keep the GraphQL' requests in the same behavior as JSON-RPC's, both return `null`, or at least let the not-found hash also return null.

@s1na @shemnon WDYT