Creating comment field w/o a comment type is allowed

Problem/Motivation

On field storage the form can be submitted even the "Comment type" dropdown shows no entries. This creates a comment field with an empty comment_type.

Steps to reproduce

1. Go to: Admin > Structure > Taxonomy
2. Create a new vocabulary
3. On the new vocabulary page click the "Manage fields" tab, then click "Add field"
4. Select "Comment" from "Add a new field" dropdown, fill a field name and press "Save and continue"
5. Note that the "Comment type" select is empty.

You cannot submit this form because the field has no value but if you go back to the "Manage fields" page (may requirrefresh), you'll note tha the field has been added.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet