use nullable types for enums

modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/AbstractCSharpCodegen.java

@@ -193,6 +193,11 @@ public abstract class AbstractCSharpCodegen extends DefaultCodegen implements Co
         }
     }
 
+    @Override
+    public String toEnumName(CodegenProperty property) {
+        return StringUtils.capitalize(property.name) + "Enum?";
+    }
+
     @Override
     public Map<String, Object> postProcessModels(Map<String, Object> objs) {
         List<Object> models = (List<Object>) objs.get("models");