graphql: add nil check in block resolver (#33225)

Add nil checks for header and block in Block resolver methods to prevent
panic when querying non-existent blocks.

graphql/graphql.go

@@ -707,6 +707,9 @@ func (b *Block) resolveHeader(ctx context.Context) (*types.Header, error) {
 	if err != nil {
 		return nil, err
 	}
+	if b.header == nil {
+		return nil, nil
+	}
 	if b.hash == (common.Hash{}) {
 		b.hash = b.header.Hash()
 	}