Option 1: Fill all empty cells with a "N/A" and then use Conditional Formatting to make the text invisible.
Or Option 2: Fill all empty cells with 0 and use an Excel setting to hide zero values.
Filling all empty cells:
- Edit → Go To... → Special ... (On Windows: Home → Editing → Find & Select → Go To Special...)
- Select "Blanks" and hit OK.
- All blank cells are selected now. Don't click anything.
- Type "N/A" or 0, and then hit Ctrl+Enter. This will insert the value into all selected cells.
Conditional Formatting to Hide "N/A"
- Format → Conditional Formatting.
- Create new rule.
- Style: Classic, and Use a formula to determine which cells to format.
- Format with: Custom Format: Font color white, no fill.
- Excel → Settings → View.
- Untick "Show zero values".