Opencast with required license metadata in series leads to problems with the plugin

[mtneug]

Describe the bug

In Opencast you can configure metadata as required. We do this for the license field in the series. Now the plugin does not set a license when creating a series. It seems that Opencast allows that in the external API. However, updates to that series fail, hindering adding/editing the series in ILIAS.

The problem might also occur with videos.

Related #51

To Reproduce
Steps to reproduce the behavior:

1. Configure license as required in the series
2. Create a new series
3. Try to update the name / take it offline/online

Expected behavior
The license is shown as a required field in ILIAS

Environment (please complete the following information):

• Plugin version: 5.0.0