fix: Support false-only variant with fallback behaviour

[lrholmes]

Description

After upgrading to the latest version of tailwind-variants, we noticed a small UI regression where a component which specified only a false variant was continually using that, despite true being specifically passed in.

It looks like this occurs because of the logic to always fall back to the false variant if the initial value isn't found:

tailwind-variants/src/index.js

Line 220 in a26d801

const value = variantObj[key] || variantObj["false"];

This PR addresses that by changing where the fallback happens. I've also added a couple of tests which cover the case that currently fails.

It's an odd case so hopefully this won't be impacting many other users! In our case, we actually realised that this API choice wasn't ideal and ended up implementing an alternative workaround. Still, the behaviour was very confusing to us for a moment and the fix seems quite simple, so decided to create a PR with the change.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Follow the Style Guide.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).

[mskelton]

Awesome! Thanks for the PR!