Going to need to see the demo in Power BI, so that all factors can be considered here.
Unfortunately the excel doesn’t really help at all.
There’s many things to consider here including the model you have, the data you have, the context of the calculation etc. So for the best help will need to see a demo example of what you are looking at.
If this is honestly as simple as turning a BLANK into a zero, this is quite easy.
All you need to do is write some logic like this.
IF( ISBLANK( measure ), 0, measure )
If you need more than this as mentioned would be helpful to actually see the model.