Field settings aren't validated

[skrolikowski]

Describe the issue

There is no validation for managing Fields through the Fieldset interface.

To reproduce

• Example: Create a new Taxonomy Field without setting a Taxonomy.

Expected behavior

The Settings column should look like: {"taxonomy":1}

Bug report checklist

• I have filled out as much information as I can
• I have detailed how the issue can be reproduced
• I have searched for existing issues and to the best of my knowledge this is not a duplicate