Wordpress sort by custom field date if it exists, otherwise sort by pub date.

sort-by-custom-field-and-non-custom-field.php

<?php

$paged = get_query_var('paged', 1);
$cat   = get_query_var('cat', '');

$args = [
    'post_type'   => 'post',
    'post_status' => 'publish',
    'paged'       => $paged,
    'cat'         => $cat,
    'meta_query'  => [
        'relation'     => 'OR',
        'event_clause' => [
            'key'     => 'end_date',
            'value'   => date('Ymd', strtotime('+1 week')),
            'compare' => '<=',
            'type'    => 'DATE',

        ],
        [
            'key'     => 'end_date',
            'compare' => 'NOT EXISTS',
        ],
    ],
    'orderby'     => 'event_clause date',
    'order'       => 'DESC',
];

$query = new WP_Query($args);