fix: handle single values in multi select relation widget

[demshy]

Summary
This should patch up a bug that was introduced here: #7161
There is an edge case that we use for our client where relation widget is collapsed within an list and if the widget wasn't displayed right away it would populate the value as if it was a single value field and after saving the format would be corrupted.

Test plan
Add the following field to the list of authors in config.yml

- { name: 'posts', widget: relation, collection: posts, multiple: true, search_fields: [ "title" ], display_fields: [ "title" ], value_field: "{{slug}}", filters: [ {field: "draft", values: [false]} ] }

In settings > authors add a single related post, publish (so far so good), close and open authors again. The value would be blank before this patch but is correctly populated now.

Checklist

Please add a x inside each checkbox:

• I have read the contribution guidelines.

A picture of a cute animal (not mandatory but encouraged)