Add null coalescing operator for php8.1 compatibility

[chris-pook]

Due to the core RateRequest::getDestPostcode() function returning null in some circumstances this ensures a php8.1 compatible value is supplied to the trim function.

[HRR1337]

Veryfied the pullrequest. It works :-)

[wsajosh]

Thanks @chris-pook, we've addressed this in the latest version