Web Forms :: Insert NULL Value In Database When TextBox Is Blank

below is House_info table in database

Id Behtop Service1 Service2

1 1111 Estate Null
2 2222 Home Home1

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:

Id Behtop Service1 Service2

1 1111 Estate
2 2222 Home Home1

I want if users don't enter any thing in text box it insert in table "NULL".How I can do it?