This will check if upsells are active on the page and if they are none will show the realted products

woocommerce.php

<?php

// REMOVE RELATED IF UPSELLS
remove_action( 'woocommerce_after_single_product_summary', 'woocommerce_upsell_display', 15 );
remove_action( 'woocommerce_after_single_product_summary', 'woocommerce_output_related_products', 20 );

add_action( 'woocommerce_after_single_product_summary', 'related_upsell_products', 15 );
function related_upsell_products() {
	global $product;

	if ( isset( $product ) && is_product() ) {
		$upsells = $product->get_upsells();

		if ( sizeof( $upsells ) > 0 ) {
			woocommerce_upsell_display();
		} else {
			// woocommerce_upsell_display();
			woocommerce_output_related_products();
		}
	}
}