fix: fix search input when value is prepopulated

[nolanlawson]

In some cases (notably with Preact it seems), the <input type="search"> value may be prepopulated (due to cached DOM nodes being re-hydrated, I suppose). In those cases, you can cause a bug if you do e.g.:

1. Type "horse" into the search
2. Press enter to choose the 🐴 emoji
3. Reopen the picker (can't be reproduced in the storybook, but can be reproduced in Pinafore)
4. The search input is prepopulated, but the search results are empty

This fixes that, by performing a search if the input happens to contain any text.

[nolanlawson]

Interestingly, this bug seems fixed in preact@10.0.0-alpha.1 (the search input is not pre-populated), but since it's a small amount of code, it seems worthwhile to do what we can to avoid an outright bug.