Sorting columns with numbers and NULL string

pioneer903

Some of my cells do not have any value, and if I sort them, the numbers are sorted properly :

2
4
33
121

I need to display NULL instead of an empty sell. For that, I just check if the value of my data is empty, I assign a string NULL to it, but in this case, sorting interprets my numbers as strings and sorts them in the following order:

121
2
33
4

Is there a way to set another condition so that sort function interprets NULL as a negative number?
My tables are different every time, so I will not be able to specify sorting type for a specific column, it just has to interpret NULL as a negative.
Or maybe there is another solution that I didn't think of?

Thank you