Revert type strictness regression

PHP 7.0 doesn't support nullable types, so we can't type-hint this as ?int. If we declare it as int, users can't explicitly pass NULL.

src/Binary.php

@@ -69,7 +69,7 @@ public static function safeStrlen(string $str): int
     public static function safeSubstr(
         string $str,
         int $start = 0,
-        int $length = null
+        $length = null
     ): string {
         if ($length === 0) {
             return '';