WP_STRIPE_KEY is undefined at plugin activation

It's only a warning until the API options are configured, but when first activated we get the following annoying notice message:


This is because the constant is only defined after the api options have been set

```
if ( ! empty( $options['stripe_api_switch'] ) ) {

if ( $options['stripe_api_switch'] === 'Yes') {
Stripe::setApiKey( $options['stripe_test_api'] );
define( 'WP_STRIPE_KEY', $options['stripe_test_api_publish'] );

} else {
Stripe::setApiKey( $options['stripe_prod_api'] );
define( 'WP_STRIPE_KEY', $options['stripe_prod_api_publish'] );
}
}
```

Start with the plugin activation path containing the shown API-options conditional and the WP_STRIPE_KEY definition. Reproduce activation before API options are configured, then verify the notice no longer appears while configured test and production settings still define the key correctly.

Written by the indexing model from the issue text.