Fix nullifying a multi-valued complex property

Pre-requisites: define a new document type with a multi-valued complex property

Steps to reproduce:

1. Create a documentof the previous type and set the Multi-valued field with at least 1 value
2. Edit the File document
3. Remove all values from the multivalued complex field
4. Save
5. Observe that the multivalued complex has still the same value(s)

Expected behavior:

The multivalued complex field should be empty after removing the value.

Example of PUT request which fails:

PUT https://nightly.nuxeo.com/nuxeo/api/v1/id/403e037c-f595-4369-ab80-5a0d70ceaea0

{
  "entity-type": "document",
  "uid": "403e037c-f595-4369-ab80-5a0d70ceaea0",
  "properties": {
    "multidoc:comp": []
  }
}