Fix panic on empty blockquotes

While renderer previously assumed there would always be a single
paragraph inside the blockquote, there sometimes can be either more or
none.

See #5.

internal/gemini/renderer.go

@@ -86,9 +86,17 @@ func (r Renderer) blockquote(w io.Writer, node *ast.BlockQuote, entering bool) {
 	// TODO: Renderer.blockquote: needs support for subnode rendering;
 	// ideally to be merged with paragraph
 	if entering {
-		if para, ok := node.Children[0].(*ast.Paragraph); ok {
-			w.Write(quotePrefix)
-			r.text(w, para)
+		if node := node.AsContainer(); node != nil {
+			for _, child := range node.Children {
+				w.Write(quotePrefix)
+				// assume children would be paragraphs
+				r.text(w, child)
+				// double linebreak to ensure Gemini clients don't merge
+				// quotes; gomarkdown assumes separate blockquotes are
+				// paragraphs of the same blockquote while we don't
+				w.Write(lineBreak)
+				w.Write(lineBreak)
+			}
 		}
 	}
 }
@@ -137,6 +145,7 @@ func (r Renderer) paragraph(w io.Writer, node *ast.Paragraph, entering bool) (no
 		}
 		linksOnly := func() bool {
 			for _, child := range children {
+				// TODO: simplify
 				if _, ok := child.(*ast.Link); ok {
 					continue
 				}
@@ -329,7 +338,6 @@ func (r Renderer) RenderNode(w io.Writer, node ast.Node, entering bool) ast.Walk
 	switch node := node.(type) {
 	case *ast.BlockQuote:
 		r.blockquote(w, node, entering)
-		noNewLine = false
 	case *ast.Heading:
 		r.heading(w, node, entering)
 		noNewLine = false