unit tests on empty cells

[samthiriot]

test:

• empty cells if fi, di
• empty cells due to pdi, pdj
• empty cells due to pij

[samthiriot]

• [ ] empty cells if fi, di
• [x] empty cells due to pdi, pdj
• [x] empty cells due to pij

[samthiriot]

covered, except for fi/fy tracked in another issue