FIX: don't use fallback key as button text

[awesomerobot]

When a non-US locale is the site's default, and the user chooses a US locale, the button would render the fallback key when no translation is set:

Before:

After:

So I've added a check for the default key, which should be considered false.

Originally reported here: https://meta.discourse.org/t/bug-with-search-banner-search-button-text-shown-in-search-banner-theme-component/273628

I think this is because the translation is being used in an unexpected way as a pseudo-setting (i.e., if it's empty, don't render the button)

[CvX]

btw, better yet - there's a translatedFallback option to i18n.t() where you can specify the fallback, and in this case I think you could use false as the value

[awesomerobot]

hmm unfortunately that doesn't seem to work in this case, I can get translatedFallback to work if I use a string, but using false or an empty string returns the translation key again