New NOT NULL column on existing table

Script:

if not exists ( select 1 from sys.tables t
                join sys.columns c on c.object_id = t.object_id
                where t.name = 'PushInterfaceRESTLog' and c.name = 'UserType')
BEGIN
    ALTER TABLE PushInterfaceRESTLog 
    ADD UserType varchar(100) NULL 
END
GO

if exists ( select 1 from sys.tables t
            join sys.columns c on c.object_id = t.object_id
            where t.name = 'PushInterfaceRESTLog' and c.name = 'UserType')
BEGIN
    UPDATE PushInterfaceRESTLog SET UserType = 'Vendor'
    
    ALTER TABLE PushInterfaceRESTLog 
    ALTER COLUMN UserType varchar(100) NOT NULL  
END
GO