fix: migration erroring when there is empty cell in table (#1223)

[![PR App][icn]][demo] | Fix RM-XYZ
:-------------------:|:----------:

## 🧰 Changes

Tables like:
```md
| Name | Date |
| ---- | ---- |
| test |      |
```
were throwing an error when migrating because we tried to access an
attribute .type whilst content is not the expected object.

This PR just does early exit when content is empty.

## 🧬 QA & Testing

- [Broken on production][prod].
- [Working in this PR app][demo].


[demo]: https://markdown-pr-PR_NUMBER.herokuapp.com
[prod]: https://SUBDOMAIN.readme.io
[icn]:
https://user-images.githubusercontent.com/886627/160426047-1bee9488-305a-4145-bb2b-09d8b757d38a.svg

Author: eaglethrost

__tests__/migration/tables.test.ts

@@ -1,6 +1,21 @@
 import { migrate } from '../helpers';
 
 describe('mdx migration of tables', () => {
+  it('compiles tables with empty cells', () => {
+    const md = `
+| Name | Date |
+| ---- | ---- |
+| test |      |
+`;
+    const mdx = migrate(md);
+    expect(mdx).toMatchInlineSnapshot(`
+      "| Name | Date |
+      | ---- | ---- |
+      | test |      |
+      "
+    `);
+  });
+
   it('compiles tables with newlines and inline code', () => {
     const md = `
 [block:parameters]

processor/transform/tables-to-jsx.ts

@@ -34,6 +34,9 @@ const visitor = (table: Table, index: number, parent: Parents) => {
         ? (cell.children[0] as unknown as Paragraph).children[0]
         : cell.children[0];
 
+    // If cell is empty there's nothing to visit
+    if (!content) return EXIT;
+
     // @note: Compatibility with RDMD. Ideally, I'd put this in a separate
     // transformer, but then there'd be some duplication.
     visit(cell, 'break', (_, breakIndex, breakParent) => {