Using a drop down box - if 'all' or 'blank' is chosen then include all

Hello...

 

Please help, I'm lost. 

 

I have a sheet with a lot of countifs and sumifs which gives a summary of performance of different people based on set criteria. 

 

I also have a drop down box so you can choose which month you want to look at - and this is all working fine.

 

What I can't figure out is how to get the summary table to include everything is the drop down box is blank or says 'all'.

 

Hope this makes sense????

 

"=COUNTIFS(Sheet1!A:A,$D$3,Sheet1!B:B,$D$4,Sheet1!E:E,Summary!B8,Sheet1!G:G,Sheet2!D$7)"

 

Where D3 and D4 are month and year.

 

When they're blank I want the countif to include everything ???