Content blocks in VE should work if field is not a multiple

When editing a document with a choice-field (content-block) in the VisualEditor, it is required to be marked as a multiple, otherwise, it is impossible to add a value.

This is different from the behavior in the ContentPerspective, wherein in such a case, the user can still add a single field.

Expected:

• add a content-block field that is not a multiple to a document
• edit the document in the VisualEditor
• add a single content-block

Actual

• the user can not add a single content-block