Fix 'Invalid type' on empty list item

[ohtake]

If mdast has an empty children, strip-markdown raises Invalid type: undefined.
The minimum example is - (hyphen and space).

// Some attributes are removed for simplicity
{ type: 'root',
  children:
   [ { type: 'list',
       children:
        [ { type: 'listItem',
            children: [] } ], }, ], }

Error: Invalid type: undefined
 at one (node_modules\strip-markdown\index.js:39:11)
 at Array.map (native)
 at all (node_modules\strip-markdown\index.js:66:36)
 at one (node_modules\strip-markdown\index.js:48:23)

[wooorm]

Thanks again, @ohtake :)