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

app/Models/Supplier.php

@@ -4,10 +4,11 @@ namespace App\Models;
 
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Notifications\Notifiable;
 
 class Supplier extends Model
 {
-    use HasFactory;
+    use HasFactory, Notifiable;
 
     protected $fillable = [
         'supplier_code', 'name', 'category',