How to force to show empty areas with ggplot2 faces?

I create field diagrams for two datasets in R + ggplot2, and I use graphs to print a graph grid based on two factors. So far, so good, the fact is that one of the two data sets completely misses the values ​​for one of the levels of one factor, so I get a 5 x 5 grid and a 5 x 4 grid, but these grids are for comparison with the eyes, and therefore I I would like to show empty graphs for missing values. Is it possible?

Thanks Tommaso