Fix for “undefined NaN” date issue in date range facet calendar

<?php
/** this error occurs when there are no available dates for the calendar picker
 ** to display, changing from 0 to "" fixes the calendar but 
 ** actual dates could be used for min/max if preferred, see
 ** https://facetwp.com/help-center/developers/hooks/output-hooks/facetwp_render_output/#customize-a-date-range-facets-date-picker-range
 ** all 4 of these checks may not be needed if both dates are
 ** to configured to display
 **/

// Replace "bad_dates" with the name of your Date Range facet
add_filter( 'facetwp_render_output', function( $output, $params ) {

    if ( isset( $output['settings']['bad_dates'] ) ) {
        if ( 0 == $output['settings']['bad_dates']['range']['min']['minDate'] ) {
            $output['settings']['bad_dates']['range']['min']['minDate'] = ''; // start date min
        }
        if ( 0 == $output['settings']['bad_dates']['range']['min']['maxDate'] ) {
            $output['settings']['bad_dates']['range']['min']['maxDate'] = ''; // start date max
        }
        if ( 0 == $output['settings']['bad_dates']['range']['max']['minDate'] ) {
            $output['settings']['bad_dates']['range']['max']['minDate'] = ''; // End date min
        }
        if ( 0 == $output['settings']['bad_dates']['range']['max']['maxDate'] ) {
            $output['settings']['bad_dates']['range']['max']['maxDate'] = ''; // End date max
        }
    }

    return $output;
  }, 10, 2 );