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/testdata/snapshots/TestGenerate-Omitempty.graphql-Omitempty.graphql.go

@@ -148,19 +148,19 @@ func (v *UserQueryInput) UnmarshalJSON(b []byte) error {
 }
 
 type __premarshalUserQueryInput struct {
-	Email string `json:"email"`
+	Email string `json:"email,omitempty"`
 
-	Name string `json:"name"`
+	Name string `json:"name,omitempty"`
 
 	Id testutil.ID `json:"id"`
 
-	Role Role `json:"role"`
+	Role Role `json:"role,omitempty"`
 
-	Names []string `json:"names"`
+	Names []string `json:"names,omitempty"`
 
-	HasPokemon testutil.Pokemon `json:"hasPokemon"`
+	HasPokemon testutil.Pokemon `json:"hasPokemon,omitempty"`
 
-	Birthdate json.RawMessage `json:"birthdate"`
+	Birthdate json.RawMessage `json:"birthdate,omitempty"`
 }
 
 func (v *UserQueryInput) MarshalJSON() ([]byte, error) {