Fix loss of sub list value when adding a new entry on a complex multivalued field

Steps to reproduce :

• create a multivalued complex fields, one of the sub-field is also multivalued
• create a document with several entries for the complex field
• switch to edit mode
• for the multivalued sub-field, add several entries
• add a new row for the complex field
  ==> the last value typed for the sub-field is removed