src/Entity/Warranty.php

@@ -149,7 +149,7 @@ class Warranty
         return $this->id;
     }
 
-    public function setSerial($serial)
+    public function setSerial($serial = null)
     {
         $this->serial = $serial;
         return $this;

src/Service/WarrantyHandler.php

@@ -73,8 +73,11 @@ class WarrantyHandler
         }
 
         // set and save values
-        $warranty->setSerial($serial)
-            ->setPlateNumber($plate_number)
+        if (trim($serial) == '')
+            $warranty->setSerial(null);
+        else
+            $warranty->setSerial($serial);
+        $warranty->setPlateNumber($plate_number)
             ->setFirstName($first_name)
             ->setLastName($last_name)
             ->setMobileNumber($mobile_number)