Using System.Text.Json for C# client generation causes warnings in combination with Nullable Reference Types

[kev-andrews]

The generated JsonInheritanceConverter has two NRT warnings that could be surpressed with #pragma to prevent build warnings:

#pragma warning disable 8602 // Disable "CS8602 Dereference of a possibly null reference"
#pragma warning disable 8600 // Disable "CS8600 Converting null literal or possible null value to non-nullable type"

[kev-andrews]

PR #4964