Nil `Operation` field in the Operation Context

### What happened?

I am using the `oc := graphql.GetOperationContext(ctx)` in a middleware in `AroundResponses` function. The `oc.Operation` field is nil when there are some validation errors.

Error details:
- Rule: `NoUnusedFragments`
- Extensions.Code: `GRAPHQL_VALIDATION_FAILED`
- Message: `Fragment \"...\" is never used.`

### What did you expect?

I expect the `Operation` filed to not be nil.

### Minimal graphql.schema and models to reproduce

### versions
- `go run github.com/99designs/gqlgen version`? gqlgen v0.17.24
- `go version`? Go 1.19