Allow null tokens and provide immediate failure responses

[Firehed]

While a null token during auth or registration will never work, allowing it through can result in some ergonomic improvements - especially when using vanilla PHP without a framework:

Before:

$token = $_POST['token'] ?? null;
if ($token === null) {
  // do something
}
try {
  $auth = $snapAuth->verifyAuthToken($token);
} catch (\SnapAuth\ApiError $e) {
  // do something
}

After:

try {
  $auth = $snapAuth->verifyAuthToken($_POST['token'] ?? null);
} catch (\SnapAuth\ApiError $e) {
  // do something
}