Using sum formulas with blank cells

Hello.

Long story short, I'm summing some specific cells which are empty, but formatted as a duration (0:00 meaning 0 hours 0 minutes). I need to get 0:00 as a result, but instead I'm getting 0.

This is problematic for me, because in the same spreadsheet I'm summing some duration times that actually contain some data with mentioned zeroes. As a result, there's an error in the cell because of cell format.

Any ideas how to solve the problem without filling blank cells with "0"?