How to include variable null check inside the WHERE clause using tkinter?

Question

Let's say that I have four variables named a,b,c,d which I am getting as input from the user and I want to have such query in my application:

Select * 
from TABLE 
where (TABLE.FIELD = 'a' OR a = null) 
AND (TABLE.FIELD = 'b' OR b = null) ...

Is there a way to include the null check for the variable inside the where clause? Otherwise I need to create if checks for all of these scenarios. I wanted to use @SET before the select but I think Tkinter does not allow it?

Can someone help me out with this?

Thanks in advance