Updates clear all the item entity relationship fields

[ikx94]

Describe the bug
When you do an update to an entity, either via SDK or REST API, all of its belongsTo fields get emptied.

To Reproduce
Steps to reproduce the behavior:

For example, when having these 2 entities:

SignType:
  seedCount: 5
  namePlural: Sign Types
  mainProp: name
  properties:
    - { name: name, type: text, validation: { isNotEmpty: true } }

Sign:
  seedCount: 3
  mainProp: friendlyName
  properties:
    - { name: friendlyName, type: text }
    - { name: fabricationDate, type: date }
    - {
        name: photo,
        type: image,
        options:
          {
            sizes:
              {
                small: { width: 500, height: 500 },
                large: { width: 1920, height: 1080 },
              },
          },
      }
  belongsTo:
    - { name: signType, entity: SignType, eager: true }

If a Sign has a signType, and you then do .update or update it through REST API, then that value will be cleared. Same thing happens if it has other relationships.

I've tried with eager set to false, and with set to true, and it still happens.

So currently from the frontend I need to add all of the related fields and add them as hidden inputs and send them in my form data.

Expected behavior
Since it's an update, no values should be cleared, unless they are sent and set to "" or null.