[AspDotNet] Add nullable to model fields

[JFCote]

The already existing nullableReferenceType added support for nullable parameters, but not for the fields inside of the models which was problematic.

With this simple change, the fields are now nullable as they should be.

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@mandrean (2017/08) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05) @iBicha (2023/07)

[JFCote]

@wing328 Small improvements to support nullable type better.

[douglas-amici]

@JFCote / @wing328, is it intended behaviour that with nullableReferenceTypes all fields will be output as nullable?