v4 API: Purchases: SKU and Required / Optional Fields

[n7studios]

Summary

• Adds sku to the products array for testing, as this is supported in the API (docs, slack)
• Reorders create_purchase method parameters to ensure required parameters listed first
• Removes blank and null parameters prior to calling the create purchase endpoint

Testing

• testCreatePurchaseWithInvalidEmailAddress: Test that a ClientException is thrown when an invalid email address is specified
• testCreatePurchaseWithBlankTransactionID: Test that a ClientException is thrown when no transaction ID defined
• testCreatePurchaseWithNoProducts: Test that a ClientException is thrown when no products are defined

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• The code passes when running PHPStan
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)