DeepLinkResource always containing an empty object for custom_params leads to issues with Moodle

[mst-kialo]

Hello,

thank you for your work on pylti1.3.
We have noticed that the DeepLinkResource sets an empty object as default for _custom_params.
This however seems to lead to issues with Moodle as apparently it cannot deal with empty objects in the custom_params claim. As far as we are aware that claim is optional as per the spec. Is there a specific reason why it is not simply set to None as default?

[hmoffatt]

I have a pull request for this here: #113