Fix 'if' function with only 2 arguments

This had a similar problem as when null values were passed explicitly
before the fix for bug 1709620.

Change-Id: I02fb4c8815bc24d4c0c339022aa857dc344d58e9
Story: 2011108
Task: 49921

heat/engine/function.py

@@ -243,7 +243,9 @@ class Macro(Function, metaclass=abc.ABCMeta):
         if isinstance(self.parsed, Function):
             return self.parsed.__reduce__()
         if self.parsed is None:
-            return lambda x: None, (None,)
+            return type(None), tuple()
+        if self.parsed is Ellipsis:
+            return type(Ellipsis), tuple()
         return type(self.parsed), (self.parsed,)
 
     def _repr_result(self):

heat/tests/test_hot.py

@@ -165,8 +165,9 @@ class HOTemplateTest(common.HeatTestCase):
     """Test processing of HOT templates."""
 
     @staticmethod
-    def resolve(snippet, template, stack=None):
+    def resolve(snippet, template, stack=None, nullable=False):
-        return function.resolve(template.parse(stack and stack.defn, snippet))
+        return function.resolve(template.parse(stack and stack.defn, snippet),
+                                nullable=nullable)
 
     @staticmethod
     def resolve_condition(snippet, template, stack=None):
@@ -1445,6 +1446,15 @@ class HOTemplateTest(common.HeatTestCase):
         resolved = self.resolve(snippet, tmpl, stack)
         self.assertIsNone(resolved)
 
+    def test_if_empty_return(self):
+        snippet = {'if': [False, 'value_if_true']}
+        # when condition is true, if function resolve to value_if_true
+        tmpl = template.Template(hot_wallaby_tpl_empty)
+        stack = parser.Stack(utils.dummy_context(),
+                             'test_if_empty_return', tmpl)
+        resolved = self.resolve(snippet, tmpl, stack, nullable=True)
+        self.assertIs(Ellipsis, resolved)
+
     def test_if_using_condition_function(self):
         tmpl_with_conditions = template_format.parse('''
 heat_template_version: 2016-10-14
@@ -1509,6 +1519,29 @@ resources:
                          self.stack.state)
         self.assertEqual('', self.stack['AResource'].properties['Foo'])
 
+    def test_if_referenced_by_resource_empty(self):
+        tmpl_with_conditions = template_format.parse('''
+heat_template_version: wallaby
+conditions:
+  create_prod: False
+resources:
+  AResource:
+    type: ResourceWithPropsType
+    properties:
+      Foo:
+        if:
+          - create_prod
+          - "prod"
+''')
+        tmpl = template.Template(tmpl_with_conditions)
+        self.stack = parser.Stack(utils.dummy_context(),
+                                  'test_if_referenced_by_resource_null', tmpl)
+        self.stack.store()
+        self.stack.create()
+        self.assertEqual((parser.Stack.CREATE, parser.Stack.COMPLETE),
+                         self.stack.state)
+        self.assertIsNone(self.stack['AResource'].properties['Foo'])
+
     def test_if_invalid_args(self):
         snippets = [
             {'if': ['create_prod', 'one_value']},