Don't throw null argument exception for non-required reference types when nrt is enabled

Author: alec-petersen

modules/openapi-generator/src/main/resources/csharp/libraries/generichost/JsonConverter.mustache

@@ -276,9 +276,18 @@
             {{/allVars}}
             {{#allVars}}
             {{^isNullable}}
+            {{#required}}
             if ({{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}.IsSet && {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}.Value == null)
                 throw new ArgumentNullException(nameof({{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}), "Property is not nullable for class {{classname}}.");
 
+            {{/required}}
+            {{^required}}
+            {{^nrt}}
+            if ({{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}.IsSet && {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}.Value == null)
+                throw new ArgumentNullException(nameof({{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}), "Property is not nullable for class {{classname}}.");
+
+            {{/nrt}}
+            {{/required}}
             {{/isNullable}}
             {{/allVars}}
             {{^vendorExtensions.x-duplicated-data-type}}
@@ -430,9 +439,18 @@
             {{^isDiscriminator}}
             {{^isNullable}}
             {{#vendorExtensions.x-is-reference-type}}
-            if ({{^required}}{{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}}Option.IsSet && {{/required}}{{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}} == null)
+            {{#required}}
+            if ({{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}} == null)
                 throw new ArgumentNullException(nameof({{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}}), "Property is required for class {{classname}}.");
 
+            {{/required}}
+            {{^required}}
+            {{^nrt}}
+            if ({{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}}Option.IsSet && {{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}} == null)
+                throw new ArgumentNullException(nameof({{#lambda.camelcase_sanitize_param}}{{classname}}{{/lambda.camelcase_sanitize_param}}.{{name}}), "Property is required for class {{classname}}.");
+
+            {{/nrt}}
+            {{/required}}
             {{/vendorExtensions.x-is-reference-type}}
             {{/isNullable}}
             {{/isDiscriminator}}