I never use data table, so this is just what I found be experimenting.
The dialog for formatting data tables has no provision for formatting the
numbers
I made some data with B5 as a blank cell and generated a chart for it. The
data table in the chart had the entry for B5 blank. But if I had a formula
like =IF(test,"", number) then B5 might look blank but a 0 shows in the data
table.
However a formula as =IF(test, NA(), number) gives a blank in the data
table
If you do not like the look of #N/A in the data series, then use conditional
formatting in the form =ISNA(cell ref) and make the font the same colour as
the cell background to hide it.
best wishes