fix(csharp): guard array-of-enum deserialization against null JSON values

Without the null check, a JSON null value for an array-of-enum property
caused the reader to advance past the null token and attempt to read the
next property as array content, desynchronizing the Utf8JsonReader.

Author: Neokil

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

@@ -252,16 +252,19 @@
                         {{^isDate}}
                         {{^isDateTime}}
                             {{#items.isEnum}}
-                            var {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}Items = new List<{{#items.isInnerEnum}}{{classname}}.{{/items.isInnerEnum}}{{{items.datatypeWithEnum}}}>();
-                            while (utf8JsonReader.Read())
+                            if (utf8JsonReader.TokenType != JsonTokenType.Null)
                             {
-                                if (utf8JsonReader.TokenType == JsonTokenType.EndArray)
-                                    break;
-                                string{{nrt?}} {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}ItemRawValue = utf8JsonReader.GetString();
-                                if ({{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}ItemRawValue != null)
-                                    {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}Items.Add({{^items.isInnerEnum}}{{{items.datatypeWithEnum}}}ValueConverter.FromStringOrDefault({{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}ItemRawValue){{/items.isInnerEnum}}{{#items.isInnerEnum}}{{classname}}.{{{items.datatypeWithEnum}}}FromStringOrDefault({{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}ItemRawValue){{/items.isInnerEnum}});
+                                var {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}Items = new List<{{#items.isInnerEnum}}{{classname}}.{{/items.isInnerEnum}}{{{items.datatypeWithEnum}}}>();
+                                while (utf8JsonReader.Read())
+                                {
+                                    if (utf8JsonReader.TokenType == JsonTokenType.EndArray)
+                                        break;
+                                    string{{nrt?}} {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}ItemRawValue = utf8JsonReader.GetString();
+                                    if ({{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}ItemRawValue != null)
+                                        {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}Items.Add({{^items.isInnerEnum}}{{{items.datatypeWithEnum}}}ValueConverter.FromStringOrDefault({{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}ItemRawValue){{/items.isInnerEnum}}{{#items.isInnerEnum}}{{classname}}.{{{items.datatypeWithEnum}}}FromStringOrDefault({{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}ItemRawValue){{/items.isInnerEnum}});
+                                }
+                                {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}} = {{>OptionProperty}}{{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}Items);
                             }
-                            {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}} = {{>OptionProperty}}{{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}}Items);
                             {{/items.isEnum}}
                             {{^items.isEnum}}
                             {{#lambda.camelcase_sanitize_param}}{{name}}{{/lambda.camelcase_sanitize_param}} = {{>OptionProperty}}JsonSerializer.Deserialize<{{{datatypeWithEnum}}}>(ref utf8JsonReader, jsonSerializerOptions){{^isNullable}}{{nrt!}}{{/isNullable}});