PHPStan: Fix level 0 errors

adminer/drivers/mysql.inc.php

@@ -1184,7 +1184,7 @@ if (!defined('Adminer\DRIVER')) {
 
 	/** Convert field in select and edit
 	* @param Field one element from fields()
-	* @return string
+	* @return ?string
 	*/
 	function convert_field($field) {
 		if (preg_match("~binary~", $field["type"])) {

adminer/drivers/pgsql.inc.php

@@ -176,9 +176,7 @@ if (isset($_GET["pgsql"])) {
 				return $return;
 			}
 
-			function warnings() {
-				return ''; // not implemented in PDO_PgSQL as of PHP 7.2.1
-			}
+			// warnings() not implemented in PDO_PgSQL as of PHP 7.2.1
 
 			function close() {
 			}