Request webViewLink when listing

[felipeelia]

I'm not quite sure how to reproduce it, but for a couple of files, we were seeing a fatal error being generated when using the package with Oneduo\NovaFileManager:

Oneduo\NovaFileManager\Entities\Entity::url(): Return value must be of type string, bool returned

It turns out the getUrl() method does return false when webViewLink is null. To work around that this PR:

1. Adds webViewLink to the list of fields required for lists
2. Make sure the getUrl() returns a string, as stated in its PHPdoc block.

[masbug]

Thank you!