Fix bugs relating to optional fields with custom (un)marshalers (#116)

## Summary:
There were a few bugs here, one of which Craig came across when pulling
the custom-unmarshaler change into webapp:
1. If you have an optional field with a custom unmarshaler, and the
   server omits the field from the response entirely (i.e. does not
   write `"myField": null`), we would still call your unmarshaler with
   an input of `[]byte(nil)`.  This is just wrong; it's our job to do
   the nil-check.  (This is the one Craig found; in practice gqlgen
   servers do not do this and I think the spec says not to although it's
   a bit fuzzy on the matter of serialization.  But in practice we have
   mocks that do it -- for required fields even! -- and it seems better
   to handle it than pass you data on which you'll probably err or even
   panic.)
2. If you have an optional field with a custom unmarshaler, and the
   server returns an explicit null (i.e. `"myField": null`), we would
   call your unmarshaler with `[]byte("null")`.  In principle the intent
   was you're supposed to implement that, as [`json.Unmarshaler`
   advises][1].  But (a) I forgot to document that, and (b) in practice
   `json.Unmarshal` [does *not* call you in that case][2], i.e. its
   advice is unnecessary.  So I think it's better for us to just match
   it, and not call you.  (And in that case I see no reason to bother
   documenting the advice.)
3. If you have an optional, `pointer: true` field with a custom
   marshaler, the reverse of (2) applies: if the pointer is nil, we
   shouldn't really call you.  (Indeed if you were a real
   `json.Marshaler` with a value-method rather than a pointer-method,
   trying to call you might panic!)  Note we don't need to explicitly
   write "null"; we just leave the `json.RawMessage` as nil, and
   `json.Marshal` [handles that][3].
4. We handle interface types effectively the same as custom
   unmarshalers, just we generate the unmarshaler.  So if you have an
   optional field with interface type, (1) would also apply there; our
   generated unmarshaler returns an error in this case.
5. While (2) doesn't apply to such optional interface fields (because we
   do the customary `if string(b) == "null"` check -- this I at least
   thought to test), if you set `pointer: true` on the field, we would
   still call the unmarshaler on the value, and it would no-op, but only
   *after* we initialized the pointer.  Put more simply, we'd return a
   non-nil pointer to nil interface, rather than a nil pointer; this is
   wrong since the whole point of `pointer: true` is you only get a
   non-nil pointer if your value is nil!  Of course, in practice there's
   little reason to use `pointer: true` on interface fields, and indeed
   this stuff gets so confusing my test was even wrong.

In this commit I fix all the bugs, by adding appropriate nil-checks to
wrap the unmarshaler-calls.  The templates are, as always, a bit
confusing, but the generated code makes it clear what changed.

Note we'll want to land this before cutting a release with custom
marshaler/unmarshaler support, because the first three bugs are
potentially quite noticeable.  (The latter two are in `v0.1.0`, but
presumably quite rare.)

[1]: https://pkg.go.dev/encoding/json#Unmarshaler
[2]: https://play.golang.org/p/Pw6zNN8trGO
[3]: https://play.golang.org/p/crTfnT7ePte

Issue: https://phabricator.khanacademy.org/D74453#inline-558571

## Test plan:
make tesc


Author: benjaminjkraft

Reviewers: csilvers, StevenACoffman, benjaminjkraft, aberkan, dnerdy, jvoll, mahtabsabet, MiguelCastillo

Required Reviewers: 

Approved By: csilvers, StevenACoffman

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/116

internal/integration/integration_test.go

@@ -175,6 +175,44 @@ func TestCustomMarshalSlice(t *testing.T) {
 	assert.Len(t, resp.UsersBornOnDates, 0)
 }
 
+func TestCustomMarshalOptional(t *testing.T) {
+	_ = `# @genqlient
+	query queryWithCustomMarshalOptional(
+		# @genqlient(pointer: true)
+		$date: Date,
+		# @genqlient(pointer: true)
+		$id: ID,
+	) {
+		userSearch(birthdate: $date, id: $id) { id name birthdate }
+	}`
+
+	ctx := context.Background()
+	server := server.RunServer()
+	defer server.Close()
+	client := graphql.NewClient(server.URL, http.DefaultClient)
+
+	date := time.Date(2025, time.January, 1, 12, 34, 56, 789, time.UTC)
+	resp, err := queryWithCustomMarshalOptional(ctx, client, &date, nil)
+	require.NoError(t, err)
+
+	assert.Len(t, resp.UserSearch, 1)
+	user := resp.UserSearch[0]
+	assert.Equal(t, "1", user.Id)
+	assert.Equal(t, "Yours Truly", user.Name)
+	assert.Equal(t,
+		time.Date(2025, time.January, 1, 0, 0, 0, 0, time.UTC),
+		user.Birthdate)
+
+	id := "2"
+	resp, err = queryWithCustomMarshalOptional(ctx, client, nil, &id)
+	require.NoError(t, err)
+	assert.Len(t, resp.UserSearch, 1)
+	user = resp.UserSearch[0]
+	assert.Equal(t, "2", user.Id)
+	assert.Equal(t, "Raven", user.Name)
+	assert.Zero(t, user.Birthdate)
+}
+
 func TestInterfaceNoFragments(t *testing.T) {
 	_ = `# @genqlient
 	query queryWithInterfaceNoFragments($id: ID!) {
@@ -322,7 +360,7 @@ func TestInterfaceListPointerField(t *testing.T) {
 	assert.Equal(t, "3", animal.Id)
 	assert.Equal(t, "Fido", animal.Name)
 
-	assert.Nil(t, *resp.Beings[2])
+	assert.Nil(t, resp.Beings[2])
 }
 
 func TestFragments(t *testing.T) {