Mongo Empty Map Bug

Spring Data MongoDB issue: spring-projects/spring-data-mongodb#5065

If a map parameter of a document is annotated with Lombok's @NonNull and @DocumentReference and is persisted with an empty map, that map will fail to deserialize in one of 2 ways:

1. If the @DocumentReference is eager, the object creation will fail when a null value is passed in the constructor.
2. If the @DocumentReference is lazy, the proxy will fail to resolve the map properly. For instance, if the getter for the map is called, and the map's values() method is called, it will return null.

Requirements

• Java 21 - Tested with Java 17 as well.
• Docker - To run Junit5 tests in TestContainers

Running the tests

Tests have been created to illustrate the problem. Run gradle test.

Due to the bug, there are two failing tests:

1. findById for parent with no @DocumentReference child

   Fails due to the ParentDocument constructor disallowing a null for the @NonNull documentReferenceChildren map.

2. Map.values for parent with no lazy @DocumentReference child

   Fails due to Map.values() method returning null when it should be populated.

Note that the equivalent tests for @DBRef work without problem.