fix(react): change button href check from key to value

[rabelloo]

Purpose

Make sure <Button href={undefined} /> renders a <button> and not an <a>.

<Button href="" /> will now also render as <button>, as will any falsey value.

Approach

Check href for value and not just the key presence.

Testing

1. Go to https://onfido.github.io/castor/?path=/docs/react-button--all-combinations, no button is disabled as they all incorrectly render as <a>
2. Go to the same story (directly from url, navigating doesn't work) in this PR preview, the first rows should render as <button>

Risks

None.

[github-actions]

🎊 PR Preview a135576 has been successfully built and deployed to https://onfido-castor-preview-pr-576.surge.sh

🕐 Build time: 190.379s

_{🤖 By surge-preview}