interopDefault fails if imported module is a non-object

[Rich-Harris]

If you have a situation like this...

// node_modules/external-dependency/index.js
module.exports = 'a string';

// main.js
import x from 'external-dependency'

...the interopDefault function searches for default inside 'a string' when the bundle runs. Need to check it's a non-null object.

[borisirota]

👍