allow interface{} as a scalar

generate/testdata/queries/EmptyInterface.graphql

@@ -0,0 +1 @@
+query EmptyInterface { getJunk }

generate/testdata/queries/EmptyInterface.graphql.go

@@ -0,0 +1,29 @@
+package test
+
+// Code generated by github.com/Khan/genqlient, DO NOT EDIT.
+
+import (
+	"github.com/Khan/genqlient/graphql"
+)
+
+type EmptyInterfaceResponse struct {
+	GetJunk interface{} `json:"getJunk"`
+}
+
+func EmptyInterface(
+	client graphql.Client,
+) (*EmptyInterfaceResponse, error) {
+	var retval EmptyInterfaceResponse
+	err := client.MakeRequest(
+		nil,
+		"EmptyInterface",
+		`
+query EmptyInterface {
+	getJunk
+}
+`,
+		&retval,
+		nil,
+	)
+	return &retval, err
+}

generate/testdata/queries/EmptyInterface.graphql.json

@@ -0,0 +1,9 @@
+{
+  "operations": [
+    {
+      "operationName": "EmptyInterface",
+      "query": "\nquery EmptyInterface {\n\tgetJunk\n}\n",
+      "sourceLocation": "testdata/queries/EmptyInterface.graphql"
+    }
+  ]
+}

generate/testdata/queries/schema.graphql

@@ -1,5 +1,7 @@
 scalar DateTime
 
+scalar Junk
+
 enum Role {
   STUDENT
   TEACHER
@@ -64,6 +66,7 @@ type Query {
   randomLeaf: LeafContent!
   convert(dt: DateTime!, tz: String): DateTime!
   maybeConvert(dt: DateTime, tz: String): DateTime
+  getJunk: Junk
 }
 
 type Mutation {