add back <none> case

crates/cli/src/subcommands/generate/csharp.rs

@@ -814,6 +814,7 @@ pub fn autogen_csharp_globals(ctx: &GenCtx, items: &[GenItem], namespace: &str)
                         "\"{reducer_str_name}\" => new Reducer.{reducer_name}(BSATNHelpers.Decode<{reducer_name}>(encodedArgs)),"
                     );
                 }
+                writeln!(output, "\"<none>\" => new Reducer.StdbNone(default),");
                 writeln!(
                     output,
                     r#"var reducer => throw new ArgumentOutOfRangeException("Reducer", $"Unknown reducer {{reducer}}")"#