Fix attestation performance API InvalidValidatorIndex error (#3503)

## Issue Addressed

When requesting an index which is not active during `start_epoch`, Lighthouse returns: 
```
curl "http://localhost:5052/lighthouse/analysis/attestation_performance/999999999?start_epoch=100000&end_epoch=100000"
```
```json
{
  "code": 500,
  "message": "INTERNAL_SERVER_ERROR: ParticipationCache(InvalidValidatorIndex(999999999))",
  "stacktraces": []
}
```

This error occurs even when the index in question becomes active before `end_epoch` which is undesirable as it can prevent larger queries from completing.

## Proposed Changes

In the event the index is out-of-bounds (has not yet been activated), simply return all fields as `false`:

```
-> curl "http://localhost:5052/lighthouse/analysis/attestation_performance/999999999?start_epoch=100000&end_epoch=100000"
```
```json
[
  {
    "index": 999999999,
    "epochs": {
      "100000": {
        "active": false,
        "head": false,
        "target": false,
        "source": false
      }
    }
  }
]
```

By doing this, we cover the case where a validator becomes active sometime between `start_epoch` and `end_epoch`.

## Additional Info

Note that this error only occurs for epochs after the Altair hard fork.

beacon_node/http_api/src/attestation_performance.rs

@@ -83,6 +83,10 @@ pub fn get_attestation_performance<T: BeaconChainTypes>(
     }
 
     // Either use the global validator set, or the specified index.
+    //
+    // Does no further validation of the indices, so in the event an index has not yet been
+    // activated or does not yet exist (according to the head state), it will return all fields as
+    // `false`.
     let index_range = if target.to_lowercase() == "global" {
         chain
             .with_head(|head| Ok((0..head.beacon_state.validators().len() as u64).collect()))