Made the check in substr() strict compare to NULL

src/Monolog/Utils.php

@@ -31,7 +31,7 @@ final class Utils
             return mb_strcut($string, $start, $length);
         }
 
-        return substr($string, $start, $length ?: strlen($string));
+        return substr($string, $start, (null === $length) ? strlen($string) : $length);
     }
 
     /**