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/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!]
+}