Fix bug for markdown rendering of blockquote (#30130)

Caused by #29984

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/markdown/transform_blockquote.go

@@ -22,10 +22,16 @@ func (g *ASTTransformer) transformBlockquote(v *ast.Blockquote, reader text.Read
 	if firstParagraph.ChildCount() < 3 {
 		return ast.WalkContinue, nil
 	}
-	node1, ok1 := firstParagraph.FirstChild().(*ast.Text)
-	node2, ok2 := node1.NextSibling().(*ast.Text)
-	node3, ok3 := node2.NextSibling().(*ast.Text)
-	if !ok1 || !ok2 || !ok3 {
+	node1, ok := firstParagraph.FirstChild().(*ast.Text)
+	if !ok {
+		return ast.WalkContinue, nil
+	}
+	node2, ok := node1.NextSibling().(*ast.Text)
+	if !ok {
+		return ast.WalkContinue, nil
+	}
+	node3, ok := node2.NextSibling().(*ast.Text)
+	if !ok {
 		return ast.WalkContinue, nil
 	}
 	val1 := string(node1.Segment.Value(reader.Source()))