TypeError: Cannot read property 'some' of undefined when `ast.comments` is missing on the AST

https://github.com/benmosher/eslint-plugin-import/blob/1377f55bc3d003519a74a89b048e11b20d53990e/src/ExportMap.js#L333
```js
ExportMap.parse = function (path, content, context) {
  // ...

  // attempt to collect module doc
  ast.comments.some(c => {
```