Use "if stack is not None" and not "if stack"

Problem:
If a nested stack has no resources, then we do not get
a "nested" link in the resource information returned by
our API.

Why?
In engine/api.py we have:

if (hasattr(resource, 'nested') and callable(resource.nested) and
            resource.nested()):
         res[rpc_api.RES_NESTED_STACK_ID] = dict(resource.nested().identifier())

The problem with this is the python definition of False, from the docs:

"instances of user-defined classes, if the class defines a __nonzero__()
 or __len__() method, when that method returns the integer zero or bool
 value False. [1]"

So if you have a stack with zero resources (len() returns 0), the
stack will be False :-O

Solution:
Use "if stack is not None:" instead of "if stack".

Change-Id: Ibdd1cb5dee6ce8e58f7d8f2586a495caded79134
Closes-bug: 1416917

heat/engine/api.py

@@ -177,7 +177,7 @@ def format_stack_resource(resource, detail=True, with_props=False,
     }
 
     if (hasattr(resource, 'nested') and callable(resource.nested) and
-            resource.nested()):
+            resource.nested() is not None):
         res[rpc_api.RES_NESTED_STACK_ID] = dict(resource.nested().identifier())
 
     if resource.stack.parent_resource:

heat/engine/resources/template_resource.py

@@ -266,7 +266,7 @@ class TemplateResource(stack_resource.StackResource):
         return self.delete_nested()
 
     def FnGetRefId(self):
-        if not self.nested():
+        if self.nested() is None:
             return six.text_type(self.name)
 
         if 'OS::stack_id' in self.nested().outputs:
@@ -280,7 +280,7 @@ class TemplateResource(stack_resource.StackResource):
             return None
 
         def _get_inner_resource(resource_name):
-            if self.nested():
+            if self.nested() is not None:
                 try:
                     return self.nested()[resource_name]
                 except KeyError:

heat/tests/test_engine_api_utils.py

@@ -207,6 +207,18 @@ class FormatTest(common.HeatTestCase):
         formatted = api.format_stack_resource(res, False)
         self.assertEqual(resource_keys, set(formatted.keys()))
 
+    def test_format_stack_resource_with_nested_stack_empty(self):
+        res = self.stack['generic1']
+        nested_id = {'foo': 'bar'}
+
+        res.nested = mock.MagicMock()
+        res.nested.return_value.identifier.return_value = nested_id
+        res.nested.return_value.__len__.return_value = 0
+
+        formatted = api.format_stack_resource(res, False)
+        res.nested.return_value.identifier.assert_called_once_with()
+        self.assertEqual(nested_id, formatted[rpc_api.RES_NESTED_STACK_ID])
+
     def test_format_stack_resource_required_by(self):
         res1 = api.format_stack_resource(self.stack['generic1'])
         res2 = api.format_stack_resource(self.stack['generic2'])