Handle omitempty correctly for slices

We were generating broken code; fixes #43.  Also fixes a bug where
applying omitempty to the entire query was broken by e597cac74c.

generate/testdata/queries/Omitempty.graphql

@@ -1,14 +1,14 @@
 # @genqlient(omitempty: true)
 query OmitEmptyQuery(
   $query: UserQueryInput,
+  $queries: [UserQueryInput],
   $dt: DateTime,
   $tz: String,
   # @genqlient(omitempty: false)
   $tzNoOmitEmpty: String,
 ) {
-  user(query: $query) {
-    id
-  }
+  user(query: $query) { id }
+  users(query: $queries) { id }
   maybeConvert(dt: $dt, tz: $tz)
   convert2: maybeConvert(dt: $dt, tz: $tzNoOmitEmpty)
 }

generate/testdata/queries/Omitempty.graphql.go

@@ -15,9 +15,10 @@ type OmitEmptyQueryResponse struct {
 	//
 	// See UserQueryInput for what stuff is supported.
 	// If query is null, returns the current user.
-	User         OmitEmptyQueryUser `json:"user"`
-	MaybeConvert time.Time          `json:"maybeConvert"`
-	Convert2     time.Time          `json:"convert2"`
+	User         OmitEmptyQueryUser      `json:"user"`
+	Users        OmitEmptyQueryUsersUser `json:"users"`
+	MaybeConvert time.Time               `json:"maybeConvert"`
+	Convert2     time.Time               `json:"convert2"`
 }
 
 // OmitEmptyQueryUser includes the requested fields of the GraphQL type User.
@@ -31,6 +32,17 @@ type OmitEmptyQueryUser struct {
 	Id mypkg.ID `json:"id"`
 }
 
+// OmitEmptyQueryUsersUser includes the requested fields of the GraphQL type User.
+// The GraphQL type's documentation follows.
+//
+// A User is a user!
+type OmitEmptyQueryUsersUser struct {
+	// id is the user's ID.
+	//
+	// It is stable, unique, and opaque, like all good IDs.
+	Id mypkg.ID `json:"id"`
+}
+
 // Role is a type a user may have.
 type Role string
 
@@ -62,26 +74,46 @@ type UserQueryInput struct {
 func OmitEmptyQuery(
 	client graphql.Client,
 	query UserQueryInput,
+	queries []UserQueryInput,
 	dt time.Time,
 	tz string,
 	tzNoOmitEmpty string,
 ) (*OmitEmptyQueryResponse, error) {
 	variables := map[string]interface{}{
-		"query":         query,
-		"dt":            dt,
-		"tz":            tz,
 		"tzNoOmitEmpty": tzNoOmitEmpty,
 	}
 
+	var zero_query UserQueryInput
+	if query != zero_query {
+		variables["query"] = query
+	}
+
+	if len(queries) > 0 {
+		variables["queries"] = queries
+	}
+
+	var zero_dt time.Time
+	if dt != zero_dt {
+		variables["dt"] = dt
+	}
+
+	var zero_tz string
+	if tz != zero_tz {
+		variables["tz"] = tz
+	}
+
 	var retval OmitEmptyQueryResponse
 	err := client.MakeRequest(
 		nil,
 		"OmitEmptyQuery",
 		`
-query OmitEmptyQuery ($query: UserQueryInput, $dt: DateTime, $tz: String, $tzNoOmitEmpty: String) {
+query OmitEmptyQuery ($query: UserQueryInput, $queries: [UserQueryInput], $dt: DateTime, $tz: String, $tzNoOmitEmpty: String) {
 	user(query: $query) {
 		id
 	}
+	users(query: $queries) {
+		id
+	}
 	maybeConvert(dt: $dt, tz: $tz)
 	convert2: maybeConvert(dt: $dt, tz: $tzNoOmitEmpty)
 }

generate/testdata/queries/Omitempty.graphql.json

@@ -2,7 +2,7 @@
   "operations": [
     {
       "operationName": "OmitEmptyQuery",
-      "query": "\nquery OmitEmptyQuery ($query: UserQueryInput, $dt: DateTime, $tz: String, $tzNoOmitEmpty: String) {\n\tuser(query: $query) {\n\t\tid\n\t}\n\tmaybeConvert(dt: $dt, tz: $tz)\n\tconvert2: maybeConvert(dt: $dt, tz: $tzNoOmitEmpty)\n}\n",
+      "query": "\nquery OmitEmptyQuery ($query: UserQueryInput, $queries: [UserQueryInput], $dt: DateTime, $tz: String, $tzNoOmitEmpty: String) {\n\tuser(query: $query) {\n\t\tid\n\t}\n\tusers(query: $queries) {\n\t\tid\n\t}\n\tmaybeConvert(dt: $dt, tz: $tz)\n\tconvert2: maybeConvert(dt: $dt, tz: $tzNoOmitEmpty)\n}\n",
       "sourceLocation": "testdata/queries/Omitempty.graphql"
     }
   ]

generate/testdata/queries/schema.graphql

@@ -99,6 +99,8 @@ type Query {
   """
   user(query: UserQueryInput): User
 
+  users(query: [UserQueryInput]): User
+
   """usersWithRole looks a user up by role."""
   usersWithRole(role: Role!): [User!]!
   root: Topic!