Fix type issue in XHubSignature::parseSignature
Summary:
This fixes the following issue:
Type error: Return value of Nasqueron\Notifications\Http\Controllers\Gate\GitHubGateController::getSignature() must be of the type string, null returned in /src/app/Http/Controllers/Gate/GitHubGateController.php:114
Reviewers: dereckson
Reviewed By: dereckson
Differential Revision: https://devcentral.nasqueron.org/D805