fix: premarshal structs get generated with omitempty tag (#267)

This PR addresses a bug described in #263

Basically, whenever a struct involves a custom genqlient binding, a
secondary "premarshal" struct gets generated.

The bug was that this "premarshal" struct was not propagating the
`omitempty` JSON tags, which was resulting in unexpected behavior.

The fix involved a few changed lines in the Go template, and a few
changes in the unit tests.

I have:
- [x] Written a clear PR title and description (above)
- [x] Signed the [Khan Academy CLA](https://www.khanacademy.org/r/cla)
- [x] Added tests covering my changes, if applicable
- [x] Included a link to the issue fixed, if applicable
- [x] Included documentation, for new features (n/a)
- [x] Added an entry to the changelog

docs/CHANGELOG.md

@@ -34,6 +34,7 @@ When releasing a new version:
 - Fixed non-deterministic generated code when querying graphql interfaces.
 - Fixed generated code when last component of package name is not a valid identifier (e.g. `"path/to/my-package"`).
 - Fixed incorrect documentation of `for` directive.
+- Fixed bug where `omitempty` JSON tags were not being correctly applied to `__premarshal` structs.
 
 ## v0.5.0