PHP 8.4 Deprecations

[sven-communitales]

There are some deprecations in the Code with PHP 8.4

Easybill\ZUGFeRD2\Model\TradePrice::create(): Implicitly marking parameter $quantity as nullable is deprecated, the explicit nullable type must be used instead

Easybill\ZUGFeRD2\Model\TradeSettlementLineMonetarySummation::create(): Implicitly marking parameter $totalAllowanceChargeAmount as nullable is deprecated, the explicit nullable type must be used instead

I could also provide a Fix and PR.