perform strict check, fixes barbushin#526

Author: bapcltd-marv

src/PhpImap/Imap.php

@@ -939,7 +939,7 @@ public static function sort(
             );
         }
 
-        if (!$result) {
+        if (false === $result) {
             throw new UnexpectedValueException('Could not sort messages!', 0, self::HandleErrors(\imap_errors(), 'imap_sort'));
         }