Allowing Null keys in JSON response of API-Platform in Symfony Project

Question

{
"@context": "/api/contexts/PackageType",
"@id": "/api/package_types",
"@type": "hydra:Collection",
"hydra:member": [
    {
        "@id": "/api/package_types/1",
        "@type": "PackageType",
        "id": 1,
        "code": "4GwAD",
        "note": "awer",
        "label": "Microlitre"
    },
    {
        "@id": "/api/package_types/2",
        "@type": "PackageType",
        "id": 2,
        "code": "AA",
        "label": "Ball"
    },
    {
        "@id": "/api/package_types/3",
        "@type": "PackageType",
        "id": 3,
        "code": "BE",
        "label": "Bundle"
   }]
}

As you can see in above response, note key is present in 1st object but not in 2nd because for 2nd data note is null in database. How can I show all keys irrespective of whether they are null or not ? I have integrated api-platform in my symfony project.