Fix exception on missing element from html-to-react

Closes GH-507.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

src/ast-to-react.js

@@ -176,7 +176,7 @@ function getNodeProps(node, key, opts, renderer, parent, index) {
       props.allowDangerousHtml = opts.allowDangerousHtml
       props.escapeHtml = opts.escapeHtml
       props.skipHtml = opts.skipHtml
-      props.element = mergeNodeChildren(node, parsedChildren)
+      props.element = node.element ? mergeNodeChildren(node, parsedChildren) : null
       break
     }
     default:

test/react-markdown.test.js

@@ -893,3 +893,11 @@ test('should support formatting at the start of a GFM tasklist (GH-494)', () =>
   const expected = '<ul><li><input type="checkbox" readonly=""/><em>a</em></li></ul>'
   expect(actual).toEqual(expected)
 })
+
+test('should not crash on weird `html-to-react` results', () => {
+  const input = '<ruby><ruby></ruby></ruby>'
+  const actual = renderHTML(<MarkdownWithHtml allowDangerousHtml children={input} />)
+  // Note: this is not conforming to how browsers deal with it.
+  const expected = '<p><ruby></ruby><ruby></ruby></p>'
+  expect(actual).toEqual(expected)
+})