Make formula cell blank until data put in source cell

[KayCruicks1234]

This formula returns a negative number if one of the data columns is blank. How do I get the formala cell to show blank until the data is polulated?

=+NETWORKDAYS(F100,H100,'FORMULA SHEET'!$B$2:$B$11)-1

The formula above in the response time column

 

[Fluff]

Hi & welcome to MrExcel.
How about

=if(count(f100,h100)=2,NETWORKDAYS(F100,H100,'FORMULA SHEET'!$B$2:$B$11)-1,"")