Add Support for Nullable Guids

[OmarElabd]

Reported by @pawciut:

In version 1.2.4 nullable Guids are incorrect. They are present at C# code as "new System.Guid?{ }" without property name to be assigned to.

example:

TypeCode = null,
new System.Guid?
{
},
TypeName = null,
Version = null,

or something like this:

Description = "Status główny",
DocumentId
Id
new System.Guid?
{
},
new System.Guid?
{
},