[CSharp] optional properties use boxed type [fix #19461]

[marcel-huber-ptv]

Fixes the bug described in #19461 by only using the primitive variants if a property is both required and non-nullable.

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

[marcel-huber-ptv]

I am not sure what to do about the failing checks. I updated docs and samples, so I believe those shouldn't fail. Maybe my change also is a breaking change to cause the other checks to fail. Please let me know, how I can proceed with this PR.

[marcel-huber-ptv]

I close this PR, because the solution @wing328 proposed in #19461 works for us, this PR is no longer needed from our side.