Clarify conditional

Co-Authored-By: Alain Schlesser <alain.schlesser@gmail.com>

Author: nylen

src/Core_Command.php

@@ -159,7 +159,7 @@ public function download( $args, $assoc_args ) {
 		list( $download_url ) = $args;
 		$from_url             = ! empty( $download_url );
 
-		if ( ! empty( $args[0] ) ) {
+		if ( $from_url ) {
 			// URL is given
 			$version      = null;
 			$from_url     = true;