When not finding anything, it would be good to display some text. This is a feature that existed in a similar module - apachesolr_autocomplete:
if (count($result) == 0) {
$result[] = array("label" => '<span class="apachesolr_autocomplete message">' . t('No suggestions.') . '</span>');
}
This is how it looks in practice:
Active
1.0
User interface
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