[14.x] Add fallback for null value in unitAmountExcludingTax method

[jayan-blutui]

Why

I updated my installation of Cashier to v14.2.0 and noticed that my invoices can't be downloaded. Did some digging around and found that the newly added unitAmountExcludingTax method in the InvoiceLineItem class doesn't handle null values coming through from the Stripe PHP library. This pull request aims to fix the issue by setting null values to 0 (int), this will ensure that values passed to Cashier::formatAmount() is always a int or string type.

Context

For some context around why I am likely to get a null value. My application charges a fixed price per website and a fixed price per user (team members). This means that a team in my application can possibly have 0 websites but always have 1 or more users, this would mean their subscription would still be active but have a $0 charge on the website line item for a given month. This situation usually only happens when a team is new or rarely has published websites.

[driesvints]

Thanks!