graphql: validate block params (#27876)

Block takes a number and a hash. The spec is unclear on what should happen in this case, leaving it an implemenation detail. With this change, we return an error in case both number and hash are passed in. 

graphql/graphql.go

@@ -1217,6 +1217,9 @@ func (r *Resolver) Block(ctx context.Context, args struct {
 	Number *Long
 	Hash   *common.Hash
 }) (*Block, error) {
+	if args.Number != nil && args.Hash != nil {
+		return nil, errors.New("only one of number or hash must be specified")
+	}
 	var numberOrHash rpc.BlockNumberOrHash
 	if args.Number != nil {
 		if *args.Number < 0 {