Usage of woocommerce_appointments_time_slots_html filter (display left spaces when no booking)

functions.php

<?php

add_filter('woocommerce_appointments_time_slots_html', 'idl_woocommerce_appointments_time_slots_html', 10, 8);
function idl_woocommerce_appointments_time_slots_html($slot_html, $slots, $intervals, $time_to_check, $staff_id, $from, $timezone, $appointment) {
	if ( empty( $intervals ) ) {
		$default_interval = 'hour' === $appointment->get_duration_unit() ? $appointment->get_duration() * 60 : $appointment->get_duration();
		$custom_interval = 'hour' === $appointment->get_duration_unit() ? $appointment->get_duration() * 60 : $appointment->get_duration();
		if ( $appointment->get_interval_unit() && $appointment->get_interval() ) {
			$custom_interval = 'hour' === $appointment->get_interval_unit() ? $appointment->get_interval() * 60 : $appointment->get_interval();
		}
		$intervals        = array( $default_interval, $custom_interval );
	}

	list( $interval, $base_interval ) = $intervals;

	$start_date = current( $slots );
	$end_date = end( $slots );

	$slots		= $appointment->get_available_slots( $slots, $intervals, $staff_id, $from );
	$slot_html	= '';

	if ( $slots ) {

		// Timezones
		$timezone_datetime = new DateTime();
		$local_time = wc_appointment_timezone_locale( 'site', 'user', $timezone_datetime->getTimestamp(), wc_time_format(), $timezone );
		$site_time = wc_appointment_timezone_locale( 'site', 'user', $timezone_datetime->getTimestamp(), wc_time_format(), wc_timezone_string() );

		// Split day into three parts
		$times = apply_filters( 'woocommerce_appointments_times_split', array(
			"morning" => array(
				"name" => __( 'Morning', 'woocommerce-appointments' ),
				"from" => strtotime("00:00"),
				"to" => strtotime("12:00"),
			),
			"afternoon" => array(
				"name" => __( 'Afternoon', 'woocommerce-appointments' ),
				"from" => strtotime("12:00"),
				"to" => strtotime("17:00"),
			),
			"evening" => array(
				"name" => __( 'Evening', 'woocommerce-appointments' ),
				"from" => strtotime("17:00"),
				"to" => strtotime("24:00"),
			),
		));

		$slot_html .= "<div class=\"slot_row\">";
		foreach( $times as $k => $v ) {
			$slot_html .= "<ul class=\"slot_column $k\">";
			$slot_html .= '<li class="slot_heading">' . $v['name'] . '</li>';
			$count = 0;

			foreach ( $slots as $slot ) {
				if ( $v['from'] <= strtotime( date( 'G:i', $slot ) ) && $v['to'] > strtotime( date( 'G:i', $slot ) ) ) {
					$selected = date( 'G:i', $slot ) == date( 'G:i', $time_to_check ) ? ' selected' : '';

					// Test availability for each slot.
					$test_availability = $appointment->get_available_appointments( $slot, strtotime( "+{$interval} minutes", $slot ), $staff_id, 1 );
					//$test_availability = 1;

					//var_dump( date( 'G:i', $slot ) );

					// Return available qty for each slot.
					if ( ! is_wp_error( $test_availability ) ) {
						if ( is_array( $test_availability ) ) {
							$available_qty = max( $test_availability );
						} else {
							$available_qty = $test_availability;
						}
					} else {
						$available_qty = 0;
					}

					// Disply each slot HTML.
					if ( $available_qty > 0 ) {
						$slot_left = " <small class=\"spaces-left\">(" . sprintf( _n( '%d left', '%d left', $available_qty, 'woocommerce-appointments' ), absint( $available_qty ) ) . ")</small>";
						$slot_locale = ( $local_time !== $site_time ) ? sprintf( __( ' data-locale="Your local time: %s"', 'woocommerce-appointments' ), wc_appointment_timezone_locale( 'site', 'user', $slot, wc_date_format() . ', ' . wc_time_format(), $timezone ) ) : '';
						$slot_html .= "<li class=\"slot$selected\"$slot_locale><a href=\"#\" data-value=\"" . date_i18n( 'G:i', $slot ) . "\">" . date_i18n( wc_time_format(), $slot ) . "$slot_left</a></li>";
					} else {
						continue;
					}

				} else {
					continue;
				}

				$count++;
			}

			if ( ! $count ) {
				$slot_html .= '<li class="slot slot_empty">' . __( '&#45;', 'woocommerce-appointments' ) . '</li>';
			}
			$slot_html .= "</ul>";
		}
		$slot_html .= "</div>";

	}

	return $slot_html;
}