ProductAttribute.ts - check if values are an array or not.

[afirlejczyk]

Short description and why it's useful

Small fix for showing attribute values (customAttributes) on Product Page.
Check if parsedValues is already an array.

Before:

After:

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility)
• I've updated the Upgrade notes and Changelog on how to port existing VS sites with this new feature

Contribution and currently important rules acceptance

• I read and followed contribution rules
• I read the TypeScript Action Plan and adjusted my PR according to it
• I read about Vue Storefront Modules and I am aware that every new feature should be a module