Even if you read the whole manual, if doing the intuitive thing results in a non-intuitive result, it's usually a bad design.

The "-,omitempty" thing is a bad design because it's a non-orthogonal combination of two features. The first part means to always treat the field as if it didn't exist, and the second part means to treat the field as if it didn't exist when it's empty; but both together means to treat the field as if it did exist with a nonsensical name ("-") when it's not empty. The second part is inverting the effect of the first part, instead of adding to it.