Describe the bug

When I create a non-participant conference speaker without a full name I get a 500 error.

To Reproduce
Steps to reproduce the behavior:

1. Sign in as admin
2. Go to admin dashaboard, conference, speakers, create a new speaker
3. Fill in "participant type": "non participant" and an empty full name
4. Save the form
5. See error

Expected behavior
To not have an error, to see that the field is required

Screenshots

Extra data (please complete the following information):

• Decidim Version: v0.28.0.dev
• Decidim installation: Nightly

Additional context

We need to add the required field to the form