Fix storage of default values in the database

When a document is created without setting a value for a field with a default value, this value is not written in the database, unless the field belongs to a schema with other dirty fields.

The consequence is that NXQL searches on this field won't return the expected documents because this field is actually empty/null.

Example;

• this NXQL query does not return the document, whereas it should

  SELECT * FROM Document WHERE schema:field = 'foo'

• this NXQL query do return the document, whereas it should not

  SELECT * FROM Document WHERE schema:field IS NULL

Not that the first query run against Elasticsearch will return the document because it's the DocumentModel with the default value which is indexed.