remove-empty-markers.php

<?php
add_filter( 'tribe_events_ajax_response', function( $response ) {
	if ( empty( $response['markers'] ) ) {
		return $response;
	}

	foreach ( $response['markers'] as $index => $marker ) {
		$zip = tribe_get_zip( $marker['event_id'] );
		if (
			empty( $marker['lat'] ) 
			&& empty( $marker['lng'] )
			&& empty( $zip )
		) {
			unset( $response['markers'][ $index ] );
		}
	}

	return $response;
} );