Fix Promise return for empty auto task graphs

lib/auto.js

@@ -157,7 +157,8 @@ export default function auto(tasks, concurrency, callback) {
     callback = once(callback || promiseCallback());
     var numTasks = Object.keys(tasks).length;
     if (!numTasks) {
-        return callback(null);
+        callback(null);
+        return callback[PROMISE_SYMBOL];
     }
     if (!concurrency) {
         concurrency = numTasks;

test/es2017/awaitableFunctions.js

@@ -308,6 +308,16 @@ module.exports = function () {
         });
         expect(calls).to.eql(['a', 'c', 'b'])
     });
+    it('should return a Promise: auto with no tasks', () => {
+        return async.auto({}).then(result => {
+            expect(result).to.equal(undefined)
+        })
+    });
+    it('should return a Promise: auto with no tasks and a concurrency limit', () => {
+        return async.auto({}, 2).then(result => {
+            expect(result).to.equal(undefined)
+        })
+    });
     it('should return a Promise: autoInject', async () => {
         const calls = []
         await async.autoInject({
@@ -323,6 +333,11 @@ module.exports = function () {
         }, 1);
         expect(calls).to.eql(['a', 'c', 'b', 'a', 'c'])
     });
+    it('should return a Promise: autoInject with no tasks', () => {
+        return async.autoInject({}).then(result => {
+            expect(result).to.equal(undefined)
+        })
+    });
 
     it('should return a Promise: compose', async () => {
         const calls = []