Adjust formula criteria when cell is blank

I am trying to do a count based on multiple criteria and I have three cells (B4,B5,B6) that specify the criteria.

=SUMPRODUCT((D13:D1937=B5)*(F13:F1937=B4)*(G13:G1937=B6)) works fine if all three cells are filled.

What I want to have is if one or more of the cells are left blank then the formula will ignore that condition. So if all three cells were blank the formula would count every cell since there is no criteria.

This might be better suited for a macro, but I am wondering if it is possible in a formula

Thanks in advance,
brian