Mailgun lists > InvalidArgumentException if description value is empty

Problem/Motivation

Although the list description is not required in the API, the SDK is throwing en exception if the description value is NULL:
InvalidArgumentException: First argument to "Http\Message\MultipartStream\MultipartStreamBuilder::createStream()" must be a string, resource or StreamInterface. in Http\Message\MultipartStream\MultipartStreamBuilder->createStream() (line 332 of /var/www/codebase/vendor/php-http/multipart-stream-builder/src/MultipartStreamBuilder.php).

Steps to reproduce

Just go to the "Mailing Lists" settings page and try to add a new list without description.

Proposed resolution

To make it easier for everyone, let's just use list title as a fallback for the description value.

Remaining tasks

Commit a fix.