graph: create user, empty givenName attribute

[rhafer]

Describe the bug

Creating a user without the "givenName" attribute returns a empty string for "givenName". It should not return that attribute at all.

curl -k -X 'POST' -u admin:admin  'https://localhost:9200/graph/v1.0/users' --data '{ "onPremisesSamAccountName": "test", "displayName":"test", "mail":"test@test.test"}' | jq .
{
  "displayName": "test",
  "givenName": "",
  "id": "da62e68f-3b08-4876-a950-780ff2f7af53",
  "mail": "test@test.test",
  "onPremisesSamAccountName": "test",
  "surname": "test"
}

expected result:

{
  "displayName": "test",
  "id": "da62e68f-3b08-4876-a950-780ff2f7af53",
  "mail": "test@test.test",
  "onPremisesSamAccountName": "test",
  "surname": "test"
}