Calling MockDatabaseReference.update with null values adds null values to database instead of deleting the entry

[Simsen3142]

Description:
when setting a value of DatabaseReference to null using update, FirebaseDatabase removes this value from the database.
MockDatabaseReference.update inserts a null value instead

How to reproduce:

test('MockDatabaseReference removes value when updating with null', () async {
  DatabaseReference reference = MockFirebaseDatabase.instance.ref('path'); // test works with FirebaseDatabase.instance.ref('path')

  await reference.update({'x':'value'});

  var snapshot = await reference.get();
  expect(snapshot.value, {'x':'value'});

  await reference.update({'x':null}); // should remove the entry

  snapshot = await reference.get();
  expect(snapshot.value, null); // this fails, actual: {'x':null}
});

[sitatec]

Thanks for reporting this, I will work on it as soon as I get some free time. PRs are also welcome :)