Fix empty struct tagged enum variant

[gustavo-shigueo]

Goal

Fix incorrect behavior of the tag attribute for structs without any fields declared with braces (struct Foo {}). These structs, when used with the tag attribute should compile and produce a type containing a single field representing the struct's tag.

This bug was revealed by a regression caused #344, which refactored internally tagged enums to take advantage of struct tagging instead of manually reimplementing the behavior.
Closes #369

Changes

How did you go about solving the problem?

Checklist

• I have followed the steps listed in the Contributing guide.
• If necessary, I have added documentation related to the changes made.
• I have added or updated the tests related to the changes made.

[gustavo-shigueo]

Hey @NyxCode, since the release of v10, we have fixed a couple of bugs related to serde_json null/boolean and now this one. Support for tokio types has also been added. I think it might be a good idea to release a version 10.1.0 to include these bug fixes

[NyxCode]

Good idea!