Trigger throws an error if pick-list is empty

trigger preventSigAccDeletion on Account (before delete) {
        for(Account acc : trigger.old){
            if( acc.Contact_type__c.contains ('Signatory')
             &&  acc.Status__c == 'Approved'){
acc.adderror('Signatory Accounts cannot be deleted. Please contact administrator if you need it to be deleted or merged');
}}}

Contact_type__c is a multi picklist

When I try to delete record where Contact_type__c is blank then the code throws following error: 

Apex trigger preventSigAccDeletion caused an unexpected exception, contact your administrator: preventSigAccDeletion: execution of BeforeDelete caused by: System.NullPointerException: Attempt to de-reference a null object: Trigger.preventSigAccDeletion: line 3, column 1

Thank you all.