fix(phpstan): improve code clarity and formatting

1. Simplified condition checks for 'firstname' and 'lastname' using the 'empty()' function.
2. Refactored the type hinting of parameters for the 'addUserToCustomerGroup',
'removeUserFromCustomerGroup', and 'setAttributesToCustomer' methods.
3. Updated the type hinting for the 'getEntityTypeTitle' method, adding a possibility to receive
null.

Related: #67

src/QUI/ERP/Customer/Customers.php

@@ -104,11 +104,11 @@ public function createCustomer($customerId, array $address = [], array $groupIds
 
             $Address->save();
 
-            if (!$User->getAttribute('firstname') || $User->getAttribute('firstname') === '') {
+            if (empty($User->getAttribute('firstname'))) {
                 $User->setAttribute('firstname', $address['firstname']);
             }
 
-            if (!$User->getAttribute('lastname') || $User->getAttribute('lastname') === '') {
+            if (empty($User->getAttribute('lastname'))) {
                 $User->setAttribute('lastname', $address['lastname']);
             }
         }
@@ -221,8 +221,8 @@ public function getCustomerGroup(): ?QUI\Groups\Group
      * @throws QUI\Users\Exception
      */
     public function addUserToCustomerGroup(
-        bool|int|string $userId,
-        QUI\Interfaces\Users\User $PermissionUser = null
+        bool | int | string $userId,
+        null | QUI\Interfaces\Users\User $PermissionUser = null
     ): void {
         if (!$userId) {
             return;
@@ -258,7 +258,7 @@ public function addUserToCustomerGroup(
      * @throws QUI\Exception
      * @throws QUI\Users\Exception
      */
-    public function removeUserFromCustomerGroup(bool|int|string $userId): void
+    public function removeUserFromCustomerGroup(bool | int | string $userId): void
     {
         $customerGroup = null;
 
@@ -287,7 +287,7 @@ public function removeUserFromCustomerGroup(bool|int|string $userId): void
      * @throws QUI\Permissions\Exception
      * @throws QUI\Users\Exception
      */
-    public function setAttributesToCustomer(bool|int|string $userId, array $attributes): void
+    public function setAttributesToCustomer(bool | int | string $userId, array $attributes): void
     {
         $User = QUI::getUsers()->get($userId);
 

src/QUI/ERP/Customer/OpenItemsList/OutputProvider.php

@@ -37,7 +37,7 @@ public static function getEntityType(): string
      * @param Locale|null $Locale $Locale (optional) - If ommitted use \QUI::getLocale()
      * @return string
      */
-    public static function getEntityTypeTitle(Locale $Locale = null): string
+    public static function getEntityTypeTitle(null | Locale $Locale = null): string
     {
         if (empty($Locale)) {
             $Locale = QUI::getLocale();