`body.upload.upload_link` is null sometimes

[jongotlin]

For an unknown reason I've got null as upload_link from Vimeo a few times.

vimeo.php/src/Vimeo/Vimeo.php

Line 586 in d3c49b7

$url = $attempt['body']['upload']['upload_link'];

This breaks the code here:

vimeo.php/src/Vimeo/Vimeo.php

Line 599 in d3c49b7

$client = $this->_tus_client_factory->getTusClient($base_url, $url);

Vimeo\Upload\TusClientFactory::getTusClient(): Argument #2 ($url) must be of type string, null given, called in /var/task/vendor/vimeo/vimeo-api/src/Vimeo/Vimeo.php on line 599

We should solve it by either throw an exception, allow null or cast to string. I have too little knowledge in this process to know what's best but I can sure open a PR if someone decides which way to go.