Fix usage of multi-valued complex field with only one subfield

This configuration is not relevant, but some customers have configured a property as multi-valued complex with only one sub field => we need to support it for people who are moving from JSF and where this configuration worked.

Steps to reproduce:

1. deploy the attached jar and start Nuxeo
2. navigate to the personal workspace
3. push the + button and choose the type "monocomplex"
4. set a title and add a new value for "signataires" : push the Add button, fill the "address" field and validate with "ok"
5. observe that the new line in the datatable is empty => you cannot see the value you set for "address" : first bug
6. Push the Create button
7. Observe that a message is displayed to indicate that the document creation failed : second bug

   {entity-type: 'exception', status: 400, message: 'Unable to deserialize property: monocomplex:signataires/-1'}
   

    

Expected behavior: the data table which contains the entries for the multi-valued complex field displays the subfield correctly, and the complex property is saved correctly when the document is created. Note that the same problem may affect the document modification.