Fixes #3899

Changes proposed in this Pull Request:

We have an issue when installing the extension, happening when registering the settings route. The Amazon Pay settings default parameters can be undefined, causing a fatal error:

An error of type E_ERROR was caused in line 102 of the file /nas/content/live/bankpolicies/wp-content/plugins/woocommerce-gateway-stripe/includes/admin/class-wc-rest-stripe-settings-controller.php. Error message: Uncaught TypeError: array_keys(): Argument #1 ($array) must be of type array, null given in /nas/content/live/bankpolicies/wp-content/plugins/woocommerce-gateway-stripe/includes/admin/class-wc-rest-stripe-settings-controller.php:102

This PR fixes that by adding fallbacks to empty arrays.

Testing instructions

Code review should be enough. Check if the tests are still passing.

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)