Allow zero (0) value fields in WP Job Manager when using WP Job Manager Field Editor

functions.php

<?php

add_filter( 'submit_job_form_validate_fields_empty_values', 'smyles_submit_job_form_validate_fields_empty_values', 99, 2);

function smyles_submit_job_form_validate_fields_empty_values( $empty_values, $meta_key ){
	// [ '', 0, 0.0, '0', null, false, [] ] is the default value (@see is_empty)
	return [ '', 0.0, null, false, [] ];
}

Alternative method for older versions or if this doesn't work:
https://gist.github.com/tripflex/fab8b3d33833ea6edb844722093e742e