Calc: What is the user-defined format value for blank cell

I would like to know what is the value of a blank cell in the user-defined format “Format code” for conditional formatting purposes.

I would like to create a format that includes blank cells and then displays something in the blank cell.

One of the current applications that I would like to do this for is that if a cell is blank after a previous entry, it will display double quotes “” to implicate that the value is the same as the previous cell.

I am currently using [>0]#;[<1]"""" which obviously only works if there is a zero or negative in the cell and does not include blank cells.