Fix implicit nullability.

Author: Crell

src/JsonException.php

@@ -61,7 +61,7 @@ public static function fromJsonError(int $jsonError, $failedValue): self
     /**
      * @param mixed $failedValue
      */
-    public function __construct(string $message = '', int $code = 0, \Throwable $previous = null, $failedValue = null)
+    public function __construct(string $message = '', int $code = 0, ?\Throwable $previous = null, $failedValue = null)
     {
         parent::__construct($message, $code, $previous);
         $this->setFailedValue($failedValue);

src/JsonParseException.php

@@ -12,7 +12,7 @@ class JsonParseException extends JsonException
      * This version forces a string for $failedValue, as that's the only thing that
      * could fail to parse, since that's all you can even try to parse.
      */
-    public function __construct(string $message = '', int $code = 0, \Throwable $previous = null, string $failedValue = '')
+    public function __construct(string $message = '', int $code = 0, ?\Throwable $previous = null, string $failedValue = '')
     {
         parent::__construct($message, $code, $previous);
         $this->setFailedValue($failedValue);