Fix multivalued complex field update when a blob is removed from a subfield

Steps to reproduce

1. Define a document type with a multivalued complex field with 2 subfields:
   • fichier: Blob
   • name: String
2. Deploy to Nuxeo
3. Create a document with this type and set 2 values for the complex field
4. Edit the document and remove the blob for the first element of the complex field. Save
5. Observe the document was not updated
   The corresponding payload for the request is almost empty

   {"entity-type":"document","uid":"7ff88b98-4263-4927-9cd7-2a85cb1fa748","properties":{}}
   

6. Edit the document, remove the blob for the first element of the complex field and also change the String field. Save
7. Observe only the String subfield has been updated
   The corresponding payload for the request is

   {
     "entity-type": "document",
     "uid": "7ff88b98-4263-4927-9cd7-2a85cb1fa748",
     "properties": {
       "complexmulti:complexe": [
         {
           "fichier": {
             "name": "SimpleTable.xlsx",
             "mime-type": "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
             "encoding": null,
             "digestAlgorithm": "MD5",
             "digest": "cfb77b8e91627e7c5006e75ce9b8dc20",
             "length": "6056",
             "data": "http://localhost:8080/nuxeo/nxfile/default/7ff88b98-4263-4927-9cd7-2a85cb1fa748/complexmulti:complexe/0/fichier/SimpleTable.xlsx?changeToken=13-0",
             "appLinks": []
           },
           "name": "fichier2"
         },
         {
           "fichier": null,
           "name": "fichier7"
         }
       ]
     }
   } 
   

Expected behavior: the blob must be removed from the complex field

It's interesting to note that the null value does not remove the blob from the property
=> there may be 2 distinct bugs here.