fix: handle both string JSON and array inputs for ListField

[Toni-hr98]

Array to String Conversion Error in ListField.php

When submitting a form with an array value for the Klaviyo List field, the following error occurs

ErrorException: Array to string conversion in ListField.php:50

Steps to Reproduce

1. Create a Klaviyo List field in Craft
2. Submit form data in a global/entry
3. Error occurs in normalizeValue() when trying to cast array to string

I've submitted a PR that handles both array and string inputs

[johnnynotsolucky]

Thanks @Toni-hr98, we're going to take a look at this soon.