Fix Billing RecurringCostClient Undefined Property

[MoLegendary]

What is the issue?

Need to use isset() instead of !is_null() to avoid "NOTICE Undefined property: stdClass::"

Link to any relevant issues.

1. https://github.com/ukfast/sdk-php/blob/master/src/Billing/RecurringCostClient.php#L74

2. https://github.com/ukfast/sdk-php/blob/master/src/Billing/RecurringCostClient.php#L78

3. https://github.com/ukfast/sdk-php/blob/master/src/Billing/RecurringCostClient.php#L82

Steps To Reproduce

Steps to reproduce the behavior:

1. GET partial data using properties=

Expected behaviour

Should not give a notice

Possible fixes

Use isset

Additional context

[MoLegendary]

I will work on this