Fix: CButton variant styling when undefined

[jrd-lewis]

This PR will fix where the CButton defaults to 'btn-undefined' when the variant is not declared.

Example, pre-PR with no declared variant attribute on a CButton component:

New Behavior:

• If the variant attribute is undeclared, it will not add 'btn-undefined' like it had been doing.
• If both the color and variant attributes are declared, it will 'btn-{variant}-{color}' like it currently does.
• If the color attribute is undeclared and the variant attribute is declared, it will 'btn-{variant}' like it currently does.

[mrholek]

@jrd-lewis, thank you for your help!