Add type check to getConfigurationMatchLevel to prevent type exception

[cewald]

Related Issues

This related to the changes of #4042 & #3939.

There are use-cases where the variant properties are null. E.g. if you load a configurable product using the product/loadProduct action without specifying a specific variant. The current, new getConfigurationMatchLevel method will drop an TypeError: Cannot read property 'id' of null exception.

I extended the method to support this behavior.

Short Description and Why It's Useful

Just in-case, prevent this error.

Which Environment This Relates To

Check your case. In case of any doubts please read about Release Cycle

• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
• RC version (https://next.storefrontcloud.io) - this is a stabilisation fix for Release Candidate of Vue Storefront. I've created branch from release branch and want to merge it back to release
• Stable version (https://demo.storefrontcloud.io) - this is an important fix for current stable version. I've created branch from hotfix or master branch and want to merge it back to hotfix

Upgrade Notes and Changelog

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

IMPORTANT NOTICE - Remember to update CHANGELOG.md with description of your change

Contribution and Currently Important Rules Acceptance

• I read and followed contribution rules