Deploying content entries with many_to_many relationship fails when field required: true

[greyskin]

Steps to reproduce:

1. Create 2 content_types with a many_to_many relationship as per the Locomotive Docs.
2. Set a required: true attribute on one (or both) of the many_to_many fields in each content_type.
3. Ensure there is at least one data entry for each content_type, referencing one another, e.g. For Content Type A:

   - Content Type A Entry 1:
   _slug: content_type_a_entry_1
   name: Content Type A Entry 1
   content_type_bs:
   - content_type_b_entry_1

   For Content Type B:

   - Content Type B Entry 1:
   _slug: content_type_b_entry_1
   name: Content Type B Entry 1
   content_type_as:
   - content_type_a_entry_1

4. Run wagon deploy [env] -d -v

If you have set required: true on both Content Types, you should receive the following error:

persisting Content Type A Entry 1 [failed] Resource invalid: content_type_a must have at least one element

If you have set required: true on only Content Type B, it will deploy Content Type A Entry 1 without issue, but you should receive the following error:

persisting Content Type B Entry 1 [failed] Resource invalid: content_type_b must have at least one element