How to make a cell with a formula show as blank?

I've had some great help from this community the past few days, so I'm back again! In my personal budget spreadsheet, I'd like areas I haven't budgeted for to be completely blank. My tables have gotten quite complicated & large with lots of categories, broken up by month and as I'm a very visual person, it would just help me to make sense of things if a certain cells stayed blank. As I'm living with family ATM, I'm not having to budget for all sorts of household expenses yet, but I still have them in my spreadsheet for the future, along with things like holidays, which I can't afford right now, but hope to in future. 

So on this simplified version of my spreadsheet as an example, how would I make D8 (for the 'Travel' category) show as blank? I saw another post where someone set a custom cell format to achieve this by having it show as blank if it's value was zero, but I where my budget & actual costs are the same, I want those to show as zero (as in D5.) I could just delete what's in the cell, but that deletes the formula too, which I'd then have to reinstate later. Can I adjust the formula to only return a result if the cells it's referencing have some value in them?

Thanks in advance for any help.