Error occurs when creating subscription without accepted mandate

[vladshut]

I received subscription from EcurringAPI with no accepted mandate. Json fragment:
"mandate_code": "ECUR-1",
"mandate_accepted": false,
"mandate_accepted_date": null,

Error occurs when creating subscription from received json response.

Error:
TypeError: DateTimeImmutable::__construct() expects parameter 1 to be string, null given
/vendor/laulamanapps/ecurring-api/src/Resource/Factory/SubscriptionFactory.php:59
/vendor/laulamanapps/ecurring-api/src/Resource/Factory/SubscriptionFactory.php:36

I have already created pull request to fix this issue. Please review it #1