Wrong return schema for `engine_getBlobsV2`

In Osaka, `engine_getBlobsV2` was introduced. According to its specification (https://github.com/ethereum/execution-apis/blob/main/src/engine/osaka.md#engine_getblobsv2), the return is:
> Array of [BlobAndProofV2](https://github.com/ethereum/execution-apis/issues/new#BlobAndProofV2) or null in case of any missing blobs.

However, in the yaml file (https://github.com/ethereum/execution-apis/blob/main/src/engine/openrpc/methods/blob.yaml#L50), the return value is not stricly defined.

I recommend revising it as:
```
schema:
oneOf:
- type: array
items:
- $ref: '#/components/schemas/BlobAndProofV2'
- type: 'null'
```

In this way, the yaml file could perfectly reflect the specifications defined in the markdown file.