Select field validation to accept Null Option

<?php
Select::make('payment_method')
                            ->nullable()
                            ->options([
                                'Cash'=>'Cash',
                                'Card'=>'Card',
                                'Cheque'=>'Cheque',
                                'BankTransfer'=>'Bank Transfer',
                                'PrePaymentPlan'=>'Pre-Payment Plan',
                                'ThirdParty'=>'Third Party', 
                                'Freebee' => 'Freebee',
                                'NA'=>'N/A']),