[csharp-netcore] Fixed csharp netcore conditional serialization

Created by: emajo

@mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05)

Using the conditional serialization flag with the csharp-netcore generator each of the properties has a correspondent flag property that determines if it has or not to be serialized when the method toJson gets called. Using the setters of the properties it works as expected, the flags are set as they have to and the serialized json is alright, but if we set the class properties with the constructor, none of the flags gets set to true, so using the toJson method we get an empty json. This solution checks in the constructor if each property is null or not, if it isn’t, the according flag gets set to true.

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