Conditional formatting of empty row that are between non empty rows

Hi,
i have a range that users need to fiil with data from a1:a1000.
sometimes they skip rows for example they fill the cell a1 and then they skip a2 and fill a3.
I need a data validation formula to fill in yellow all those empty line but not the lines that that below them there is nothing.
for example if the last filled row is 100 then data validation will fill yellow only in empty rows above 100 and not below.

thanks