Fix userid handling

MatomoTracker.php

@@ -159,7 +159,7 @@ class MatomoTracker
     public $configReferralCookieTimeout = 15768000; // 6 months
 
     // Visitor Ids in order
-    public $userId = false;
+    public $userId = null;
 
     public $forcedVisitorId = false;
 
@@ -1571,11 +1571,11 @@ public function setIp(string $ip)
      *
      * A User ID can be a username, UUID or an email address, or any number or string that uniquely identifies a user or client.
      *
-     * @param string $userId Any user ID string (eg. email address, ID, username). Must be non empty. Set to false to de-assign a user id previously set.
+     * @param string|null $userId Any user ID string (eg. email address, ID, username). Must be non empty. Set to null to de-assign a user id previously set.
      * @return $this
      * @throws Exception
      */
-    public function setUserId(string $userId)
+    public function setUserId(?string $userId)
     {
         if ($userId === '') {
             throw new Exception("User ID cannot be empty.");
@@ -1674,9 +1674,9 @@ public function getIp()
      * Returns the User ID string, which may have been set via:
      *     $v->setUserId('[email protected]');
      *
-     * @return bool
+     * @return string|null
      */
-    public function getUserId()
+    public function getUserId(): ?string
     {
         return $this->userId;
     }