Fix markdown render (#33870)

Fix #33869, some code block is moved to make it easier to test.

Added a new test

modules/markup/markdown/transform_blockquote.go

@@ -115,6 +115,9 @@ func (g *ASTTransformer) transformBlockquote(v *ast.Blockquote, reader text.Read
 
 	// grab these nodes and make sure we adhere to the attention blockquote structure
 	firstParagraph := v.FirstChild()
+	if firstParagraph == nil {
+		return ast.WalkContinue, nil
+	}
 	g.applyElementDir(firstParagraph)
 
 	attentionType, processedNodes := g.extractBlockquoteAttentionEmphasis(firstParagraph, reader)