Prevent validation on custom structs for nil values

[simitt]

According to https://github.com/elastic/go-ucfg/blob/master/validator.go#L213, the validation tag nonzero should only return an error if the configuration value is present but zero.
For custom types this is not always the case at the moment.

Eg. have following custom type and configuration raises an error even if the hosts field is not present:

type urls []*url.URL

type MyConfig struct {
  Hosts urls `config:"hosts" validate:"nonzero"`
}

[urso]

Would it make sense to use required for your use-case?

[simitt]

Not really, the value is not required, but if given it is not allowed to be zero, basically exactly what I would expect from nonzero.
We have a workaround by doing a manual validation for now, but this seems to have been introduced with one of the latest changes, as we used the tag on the custom type already.

[blakerouse]

@simitt You are correct, the issue is with the latest changes the nonzero is being called when the array is nil. Working on a fix so nonzero doesn't validate when the value is nil.