It really depends on what else your displaying posts based on when there is no end_date, but you can get all values > today or posts that do not have end_date set something like this
$meta_query = array(
'relation' => 'OR',
array(
'key' => 'end_date',
'compare' => '>=',
'value' => $today,
),
array(
'key' => 'end_date',
'compare' => 'NOT EXIST'
),
);
https://codex.wordpress.org/Class_Reference/WP_Query#Custom_Field_Parameters