Replace empty cells in range variable

I'm creating a custom function in which I also do a sumproduct so I transfer 2 ranges to the VBA code.

Sometimes the first column contains rows which don't have a value yet and then that empty cell (0 in the range, I believe) should be replaced with a default aaa value.

How can I replace the empty cells in the first range with the default value in the range within the macro?

I've tried below code setup but that doesn't work... and I seem to be unable to find any proper example on replacing values in a range in a VBA function

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example.png

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