Grail 3.1 empty list is saving as null in mongodb

Hey, I am using grails 3.1. In this version, by default mongo is using MongoCodecSession but I have convertd it to MongoSession for some reason. Now when I am saving my domain instance in mongodb, it is saving empty list as null values which causes problem is $push. Below is my saving code
```
if(commit){
instance.save(flush: true)
instances.add(instance)
}
```
Any help?

Start by reproducing the Grails 3.1 save path using MongoSession rather than the default MongoCodecSession, with a domain instance containing an empty list. Inspect how instance.save(flush: true) persists empty lists and verify whether the stored value remains an empty list so subsequent $push operations work correctly.

Written by the indexing model from the issue text.