fix: add Notifiable trait to Supplier and Customer, add null guards to notification messages

app/Models/Customer.php

@@ -4,10 +4,11 @@ namespace App\Models;
 
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Notifications\Notifiable;
 
 class Customer extends Model
 {
-    use HasFactory;
+    use HasFactory, Notifiable;
 
     protected $fillable = ['name', 'contact_person', 'email', 'phone', 'whatsapp_number', 'address', 'tax_number', 'credit_limit', 'outstanding_balance', 'is_active'];