Working with blank cells and the ISBLANK function

I am having trouble working with cells that have the calculated result of "" (quotation marks to indicate blank). Those cells contain an IF funcion that should return nothing if the given requirements are not met. E.g. in cell C1 =IF( A1>0 ; A1+B1 ; "" )

The result of this cell I want to use in a second formula. This second formula should only be calculated if the result of the first formula is not blank. So I tried this in D1:

= IF( ISBLANK(C1) ; "" ; "Some text" )

My problem now is that the function ISBLANK does not recognise the result of "" as a blank cell. I have tried to use the expression >0 instead, but then I get the warning "expected a number but got a string".

I want to use blank cells because otherwise I would have hundreds of zeros in my template which would not be very usefull.

Screenshot (German)