fix: change customer email property type to a nullable string

[edwinvdpol]

Hi 👋

After I added strict type to my PHP file, I got this error in production:

TypeError
strtolower(): Argument #1 ($string) must be of type string, null given

in this line of code:

// $mollie is an instance of Mollie\Api\Resources\Customer
if ($user->email !== strtolower($mollie->email) && filled($user->email)) {
    ....
}

This PR changes the customer email property type to a nullable string.

[sandervanhooft]

Thanks @edwinvdpol , let me verify this.

[sandervanhooft]

Hi @edwinvdpol ,

Double check to ensure we get this right: this looks like an issue raised by a static type checker, i.e. PHPStan, correct?

So the code itself is not actually throwing an exception?

[edwinvdpol]

Hi @sandervanhooft,

It did throw an exception in production after I added declare(strict_types=1); to the top of the file: