Fix Behaviour of Falsey Boolean Variants

[joe-bell]

Description

Currently, false boolean variants don't work as expected. Somewhere cva is probably considering these variants false and ignoring them, rather than actually using them

Additionally, allow disabling of defaultVariants by passing an explicit null

• Add tests to check for false boolean variants (and null values to disabled them)
• Add the actual fix for this behaviour

Additional context

Follow-up to #21

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[joe-bell]

(these tests should fail until a fix is added)