fix: a bug where empty collections would not be serialized for default values

Signed-off-by: Vincent Biret <vibiret@microsoft.com>

Author: baywet

src/Microsoft.OpenApi/Writers/OpenApiWriterExtensions.cs

@@ -141,9 +141,9 @@ public static void WriteOptionalObject<T>(
         {
             if (value != null)
             {
-                if (value is IEnumerable values && !values.GetEnumerator().MoveNext())
+                if (value is IEnumerable values && value is not JsonArray && !values.GetEnumerator().MoveNext())
                 {
-                    return; // Don't render optional empty collections
+                    return; // Don't render optional empty collections except for the Default properties which are JsonArray
                 }
 
                 writer.WriteRequiredObject(name, value, action);