widget 'single vocabulary' should save an empty value when used in a list of strings in a list of complex fields

How to reproduce:

• In Studio
  • create a new document type whose contains a list of complex fields containing a field of type String
  • in the Edition layout and view layout, use widget single vocabulary for the metadata of type String
• In a Nuxeo instance
  • Create a document with the type defined above
  • select a value for the metadata of type String and save the document
  • Edit the document, select Select a value for the metadata of type String and save the document

The metadata of type String is not empty, it still contains the value selected before the edition.