Remove button variant class when no variant set

[rmccar]

What is the context of this PR?

Fixes: #3371

The variant button class is being added to the button whether a variant is set or not because of the way the logic around adding the class is written. This PR fixes the logic.

How to review this PR

• Test an example with a button in like the access code one and check that the class isn't on the button
• Test adding a variant to the button, using both a string and multiple with an array and see that the classes are added

Checklist

This needs to be completed by the person raising the PR.

• I have selected the correct Assignee
• I have linked the correct Issue

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