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-Hasura.graphql-Hasura.graphql.go

@@ -0,0 +1,118 @@
+// Code generated by github.com/Khan/genqlient, DO NOT EDIT.
+
+package test
+
+import (
+	"github.com/Khan/genqlient/graphql"
+	"github.com/Khan/genqlient/internal/testutil"
+)
+
+type GetPokemonBoolExp struct {
+	And   []*GetPokemonBoolExp `json:"_and"`
+	Not   *GetPokemonBoolExp   `json:"_not"`
+	Or    []*GetPokemonBoolExp `json:"_or"`
+	Level *IntComparisonExp    `json:"level"`
+}
+
+// GetAnd returns GetPokemonBoolExp.And, and is useful for accessing the field via an interface.
+func (v *GetPokemonBoolExp) GetAnd() []*GetPokemonBoolExp { return v.And }
+
+// GetNot returns GetPokemonBoolExp.Not, and is useful for accessing the field via an interface.
+func (v *GetPokemonBoolExp) GetNot() *GetPokemonBoolExp { return v.Not }
+
+// GetOr returns GetPokemonBoolExp.Or, and is useful for accessing the field via an interface.
+func (v *GetPokemonBoolExp) GetOr() []*GetPokemonBoolExp { return v.Or }
+
+// GetLevel returns GetPokemonBoolExp.Level, and is useful for accessing the field via an interface.
+func (v *GetPokemonBoolExp) GetLevel() *IntComparisonExp { return v.Level }
+
+// GetPokemonResponse is returned by GetPokemon on success.
+type GetPokemonResponse struct {
+	GetPokemon []*testutil.Pokemon `json:"getPokemon"`
+}
+
+// GetGetPokemon returns GetPokemonResponse.GetPokemon, and is useful for accessing the field via an interface.
+func (v *GetPokemonResponse) GetGetPokemon() []*testutil.Pokemon { return v.GetPokemon }
+
+type IntComparisonExp struct {
+	Eq     *int   `json:"_eq"`
+	Gt     *int   `json:"_gt"`
+	Gte    *int   `json:"_gte"`
+	In     []*int `json:"_in"`
+	IsNull *bool  `json:"_isNull"`
+	Lt     *int   `json:"_lt"`
+	Lte    *int   `json:"_lte"`
+	Neq    *int   `json:"_neq"`
+	Nin    []*int `json:"_nin"`
+}
+
+// GetEq returns IntComparisonExp.Eq, and is useful for accessing the field via an interface.
+func (v *IntComparisonExp) GetEq() *int { return v.Eq }
+
+// GetGt returns IntComparisonExp.Gt, and is useful for accessing the field via an interface.
+func (v *IntComparisonExp) GetGt() *int { return v.Gt }
+
+// GetGte returns IntComparisonExp.Gte, and is useful for accessing the field via an interface.
+func (v *IntComparisonExp) GetGte() *int { return v.Gte }
+
+// GetIn returns IntComparisonExp.In, and is useful for accessing the field via an interface.
+func (v *IntComparisonExp) GetIn() []*int { return v.In }
+
+// GetIsNull returns IntComparisonExp.IsNull, and is useful for accessing the field via an interface.
+func (v *IntComparisonExp) GetIsNull() *bool { return v.IsNull }
+
+// GetLt returns IntComparisonExp.Lt, and is useful for accessing the field via an interface.
+func (v *IntComparisonExp) GetLt() *int { return v.Lt }
+
+// GetLte returns IntComparisonExp.Lte, and is useful for accessing the field via an interface.
+func (v *IntComparisonExp) GetLte() *int { return v.Lte }
+
+// GetNeq returns IntComparisonExp.Neq, and is useful for accessing the field via an interface.
+func (v *IntComparisonExp) GetNeq() *int { return v.Neq }
+
+// GetNin returns IntComparisonExp.Nin, and is useful for accessing the field via an interface.
+func (v *IntComparisonExp) GetNin() []*int { return v.Nin }
+
+// __GetPokemonInput is used internally by genqlient
+type __GetPokemonInput struct {
+	Where *GetPokemonBoolExp `json:"where"`
+}
+
+// GetWhere returns __GetPokemonInput.Where, and is useful for accessing the field via an interface.
+func (v *__GetPokemonInput) GetWhere() *GetPokemonBoolExp { return v.Where }
+
+// The query or mutation executed by GetPokemon.
+const GetPokemon_Operation = `
+query GetPokemon ($where: getPokemonBoolExp!) {
+	getPokemon(where: $where) {
+		species
+		level
+	}
+}
+`
+
+func GetPokemon(
+	client graphql.Client,
+	where *GetPokemonBoolExp,
+) (*GetPokemonResponse, error) {
+	req := &graphql.Request{
+		OpName: "GetPokemon",
+		Query:  GetPokemon_Operation,
+		Variables: &__GetPokemonInput{
+			Where: where,
+		},
+	}
+	var err error
+
+	var data GetPokemonResponse
+	resp := &graphql.Response{Data: &data}
+
+	err = client.MakeRequest(
+		nil,
+		req,
+		resp,
+	)
+
+	return &data, err
+}
+