field can be saved with an empty display_id

Problem/Motivation

If there is a restriction on the types of display that can be referenced in a field, it's possible to save broken data:

1. Select a view which doens't have any displays of that type
2. The display ID form element updates to show an empty list

The widget allows the field to be saved at this point.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes