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

generate/marshal.go.tmpl

@@ -31,9 +31,9 @@
 type __premarshal{{.GoName}} struct{
     {{range .FlattenedFields -}}
     {{if .NeedsMarshaling -}}
-    {{.GoName}} {{repeat .GoType.SliceDepth "[]"}}{{ref "encoding/json.RawMessage"}} `json:"{{.JSONName}}"`
+    {{.GoName}} {{repeat .GoType.SliceDepth "[]"}}{{ref "encoding/json.RawMessage"}} `json:"{{.JSONName}}{{if .Omitempty -}},omitempty{{end}}"`
     {{else}}
-    {{.GoName}} {{.GoType.Reference}} `json:"{{.JSONName}}"`
+    {{.GoName}} {{.GoType.Reference}} `json:"{{.JSONName}}{{if .Omitempty -}},omitempty{{end}}"`
     {{end}}
     {{end}}
 }