EdiSerializer must respect conditional indication on EdiValue attribute

when a property type is nullable and the value is `null` then there must be a check to the Mandatory indicator upon the `EdiValueAttribute` in order to decide if we will write or not the value.

Start by locating EdiSerializer's handling of nullable properties and the EdiValueAttribute.Mandatory setting. No source file or test is named in the issue, so trace the serializer entry point and existing serializer tests; done means null nullable values follow the attribute's mandatory indication.

Written by the indexing model from the issue text.