Fix fatal error related to the safe_style_css filter

[wjrosa]

Changes proposed in this Pull Request:

We are getting some fatal errors in Grafana related to the inclusion of the display CSS property as a safe style using the safe_style_css filter (includes/payment-methods/class-wc-stripe-upe-payment-gateway.php#290):

PHP Fatal error:  Uncaught TypeError: array_merge(): Argument #1 must be of type array, null given in /wordpress/plugins/woocommerce-gateway-stripe/10.0.1/includes/payment-methods/class-wc-stripe-upe-payment-gateway.php:295
Stack trace:

While the root cause is not known (probably due to another filter overriding the default value), the change in this PR is pretty safe, as it just checks if the default style is an array.

Testing instructions

Code review should be enough.

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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[diegocurbelo]

Makes sense, according to the docs $styles is expected to be a string[]

[malithsen]

Thanks for the fix. LGTM!