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

Author: kevinmichaelchen

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
 

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}}
 }

generate/testdata/queries/Hasura.graphql

@@ -0,0 +1,7 @@
+# @genqlient(pointer: true)
+query GetPokemon($where: getPokemonBoolExp!) {
+  getPokemon(where: $where) {
+    species
+    level
+  }
+}

generate/testdata/queries/schema.graphql

@@ -180,8 +180,28 @@ type Query {
   listOfListsOfListsOfContent: [[[Content!]!]!]!
   recur(input: RecursiveInput!): Recursive
   acceptsListOfListOfListsOfDates(datesss: [[[Date!]!]!]!): Boolean
+  getPokemon(where: getPokemonBoolExp): [Pokemon!]!
 }
 
 type Mutation {
   createUser(name: String!, email: String): User 
 }
+
+input getPokemonBoolExp {
+  _and: [getPokemonBoolExp!]
+  _not: getPokemonBoolExp
+  _or: [getPokemonBoolExp!]
+  level: IntComparisonExp
+}
+
+input IntComparisonExp {
+  _eq: Int
+  _gt: Int
+  _gte: Int
+  _in: [Int!]
+  _isNull: Boolean
+  _lt: Int
+  _lte: Int
+  _neq: Int
+  _nin: [Int!]
+}

generate/testdata/snapshots/TestGenerate-Hasura.graphql-Hasura.graphql.go

@@ -0,0 +1,9 @@
+{
+  "operations": [
+    {
+      "operationName": "GetPokemon",
+      "query": "\nquery GetPokemon ($where: getPokemonBoolExp!) {\n\tgetPokemon(where: $where) {\n\t\tspecies\n\t\tlevel\n\t}\n}\n",
+      "sourceLocation": "testdata/queries/Hasura.graphql"
+    }
+  ]
+}