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.json

@@ -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"
+    }
+  ]
+}