Made it possible to apply and join to empty TaskTree

Thanks satels for the report

celery_tasktree.py

@@ -61,7 +61,10 @@ def apply_and_join(self):
 def join_tree(async_result):
     """ Join to all async results in the tree """
     output = []
-    first_result = async_result.join()[0]
+    results = async_result.join()
+    if not results:
+        return output
+    first_result = results[0]
     while True:
         output.append(first_result)
         if not getattr(first_result, 'async_result', None):

tests.py

@@ -103,3 +103,8 @@ def test_push_and_pop():
     tree.push(mkdir, args=('d0/1',))
     tree.push(mkdir, args=('d0/1/2',))
     [res0, res1, res2] = tree.apply_and_join()
+
+def test_empty_task_tree():
+    tree = TaskTree()
+    results = tree.apply_and_join()
+    eq_(results, [])