public List<ObjectNode> getNullableReferences(DefinitionKey key) {
        return Collections.unmodifiableList(this.nullableReferences.getOrDefault(key, Collections.emptyList()));
    }
Getter for the nodes representing nullable references to the given type.

@param key definition key to look-up collected references for
@return nullable nodes to be populated with the schema of the given type
jsonschema-generator/src/main/java/com/github/victools/jsonschema/generator/impl/SchemaGenerationContextImpl.java