src/Mapping/ClassMetadata.php
public function getDefaultSerializedModel(): array
    {
        $out = [];
        $attributeList = $this->getAttributeList();
        if ($attributeList) {
            foreach ($attributeList as $attribute) {
                $out[$attribute->getSerializedKey()] = null;
            }
        }

        $relationList = $this->getRelationList();
        if ($relationList) {
            foreach ($relationList as $relation) {
                if ($relation->isOneToMany()) {
                    $out[$relation->getSerializedKey()] = [];
                }
            }
        }

        return $out;
    }
public function getDefaultSerializedModel(): array
    {
        $out = [];
        $attributeList = $this->getAttributeList();
        if ($attributeList) {
            foreach ($attributeList as $attribute) {
                $out[$attribute->getSerializedKey()] = null;
            }
        }

        $relationList = $this->getRelationList();
        if ($relationList) {
            foreach ($relationList as $relation) {
                if ($relation->isOneToMany()) {
                    $out[$relation->getSerializedKey()] = [];
                }
            }
        }

        return $out;
    }
return default serialize model with null value or empty array on relations

@return array<string, array|null>