You can use Data Validation to prevent entry in cells if other cells are not filled in.
Select the cells that you don't want filled in unless other cells are filled in and then click on Data Validation
Allow: Custom
Uncheck: Ignore Blank
Enter this formula (this is for A1 having to have contents. Change to suit your data)
If your cells are in a column then you can select the cell with the Data Validation and fill down just like you would any ordinary formula.
Experiment a little to see how you can adapt this to your data.
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