Steps to reproduce:

1. Go to Configuration -> Hosts -> Discovery
2. Create new Discovery
3. Add Override
4. Fill override name
5. Add one more filter row
6. Select Type of calculation as custom expression (leave empty)
7. Remove one or all filter rows
8. Try to save override

Result: error "Incorrect value for field "Custom expression": cannot be empty.". Need to add two filter rows again and change "Type of calculation"
Expected: successfully added