Insert NULL value in Database when TextBox is blank in ASP.Net

HI 

below is House_info table in database

and I have 2 Textbox in page that users can enter text on them and update House_info tables column service1 and service2

below is SP:

@Service1  nvarchar(20)=NULL,
@Service2  nvarchar(20)=NULL
as
begin
update House_info set
Service1=@Service1,Service2=@Service2

END

and behind code

 _cmd.Parameters.AddWithValue("@Service1", TxtMT1.Text);
            _cmd.Parameters.AddWithValue("@Service2", TxtMT2.Text);

problem is that if users enter text in TB and click on button it update columns data but if they don't enter any thing in textbox it enter space in Table like:

I want if users don't enter any thing in text box it insert in table "NULL"

How I can do it?

Best Regards

Neda