Force search facet to return 0 results if the the minimum character limit isn’t met

<?php

// Add this to your (child) theme's functions.php

add_filter( 'facetwp_search_query_args', function( $args ) {
    if ( strlen( $args['s'] ) < 3 ) {
        $args['post__in'] = array( 0 ); // force no results to be returned
    }
    return $args;
});