When marshaling, a struct field with the new `omitzero` option in the struct field
tag will be omitted if its value is zero. If the field type has an `IsZero() bool`
method, that will be used to determine whether the value is zero. Otherwise, the
value is zero if it is [the zero value for its type](/ref/spec#The_zero_value).

If both `omitempty` and `omitzero` are specified, the field will be omitted if the
value is either empty or zero (or both).
