DatiTrasmissione.Builder: do not require field that with default value

[siboxd]

Is your feature request related to a problem? Please describe.
When compiling DatiTrasmissione Builder constructor a recipient code is required... even if you don't have one and should put the default "0000000" code.

Describe the solution you'd like
Remove from constructor the mandatory field, initializing it with the default value 0000000;
so if someone wants to modify it, it can with appropriate setter...