Cond attribute does not support arbitrary types

[p-lindberg]

The cond attribute appears to only work with Option fields, at least when dealing with custom types. It seems reasonable to assume that any type that implements Default should support conditional parsing. Also, there's no indication that the cond attribute is not doing anything for these types - we only realised this after carefully going through the trace logs.

Switching from cond = "expression" to skip, cond = "!expression" instead seems to give the expected result for arbitrary types.