Handle empty customers array in filtering logic

internal_frontend/app/customers/page.tsx

@@ -50,6 +50,8 @@ export default function CustomersPage() {
     }, []);
 
     const filtered = useMemo(() => {
+        if(customers.length === 0) return [];
+
         return customers.filter(
             (c) =>
                 c.name.toLowerCase().includes(search.toLowerCase()) ||