fix: Handle links without children properly (#86)

* 🐛  Fix bug that prevents childMarkdownRemark from being set

* Updates after code review

Author: macdonst

src/__tests__/__fixtures__/kitchensink.md

@@ -15,6 +15,8 @@ This is a paragraph with a [link](https://example.com).
 
 <https://example.com>
 
+[](https://example.com)
+
 https://codepen.io/team/codepen/pen/PNaGbb
 
 https://codesandbox.io/s/ynn88nx9x?view=split

src/__tests__/plugin.js

@@ -41,6 +41,8 @@ describe('gatsby-remark-embedder', () => {
 
       <https://example.com>
 
+      [](https://example.com)
+
       <iframe src=\\"https://codepen.io/team/codepen/embed/preview/PNaGbb\\" style=\\"width:100%; height:300px;\\"></iframe>
 
       <iframe src=\\"https://codesandbox.io/embed/ynn88nx9x?view=split\\" style=\\"width:100%; height:500px; border:0; border-radius: 4px; overflow:hidden;\\" allow=\\"geolocation; microphone; camera; midi; vr; accelerometer; gyroscope; payment; ambient-light-sensor; encrypted-media; usb\\" sandbox=\\"allow-modals allow-forms allow-popups allow-scripts allow-same-origin\\"></iframe>

src/index.js

@@ -31,6 +31,7 @@ export default async (
     const isValidLink =
       node.type === 'link' &&
       node.title === null &&
+      node.children.length === 1 &&
       node.children[0].value === node.url;
     if (!isText && !isValidLink) {
       return;