Various fixes for DB seeding

app/Http/Controllers/CustomerController.php

@@ -56,7 +56,7 @@ public static function generateCustomerNumber()
             // DATEV standard
             $randomNumber = rand(10000, 69999);
             $customer = Customer::firstWhere('customer_nr', $randomNumber);
-            if ($customer) {
+            if ($customer || Customer::count() === 0) {
                 return $randomNumber;
             }
         }