More type safety.

Author: simensen

src/Dflydev/Hawk/Server/Server.php

@@ -182,7 +182,7 @@ public function createHeader(CredentialsInterface $credentials, Artifacts $artif
  'mac' => $this->crypto->calculateMac('response', $credentials, $responseArtifacts),
  );
 
- if ($hash) {
+ if ($hash !== null) {
  $attributes['hash'] = $hash;
  }