Fix panic on NullTime.UnmarshalJSON

[khoden]

type WithDate struct {
	Time dat.NullTime `json:"time"`
}

func TestParseNumberAsDate(t *testing.T) {
	data := `{"time":5}`
	result := &WithDate{}
	json.Unmarshal([]byte(data), result) // PANIC!
}

[JelteF]

This test should probably be added to the actual tests as well.

[khoden]

Please, accept