graphql: fix block resolving for parent field (ethereum#24191)

Fixes ethereum#24161

graphql/graphql.go

@@ -599,21 +599,18 @@ func (b *Block) BaseFeePerGas(ctx context.Context) (*hexutil.Big, error) {
 }
 
 func (b *Block) Parent(ctx context.Context) (*Block, error) {
-	// If the block header hasn't been fetched, and we'll need it, fetch it.
-	if b.numberOrHash == nil && b.header == nil {
-		if _, err := b.resolveHeader(ctx); err != nil {
-			return nil, err
-		}
+	if _, err := b.resolveHeader(ctx); err != nil {
+		return nil, err
 	}
-	if b.header != nil && b.header.Number.Uint64() > 0 {
-		num := rpc.BlockNumberOrHashWithNumber(rpc.BlockNumber(b.header.Number.Uint64() - 1))
-		return &Block{
-			backend:      b.backend,
-			numberOrHash: &num,
-			hash:         b.header.ParentHash,
-		}, nil
+	if b.header == nil || b.header.Number.Uint64() < 1 {
+		return nil, nil
 	}
-	return nil, nil
+	num := rpc.BlockNumberOrHashWithNumber(rpc.BlockNumber(b.header.Number.Uint64() - 1))
+	return &Block{
+		backend:      b.backend,
+		numberOrHash: &num,
+		hash:         b.header.ParentHash,
+	}, nil
 }
 
 func (b *Block) Difficulty(ctx context.Context) (hexutil.Big, error) {