API tolerate None environment string

If an API request has a None environment argument, we should not
reject the request, but treat it the same as an empty environment.

Change-Id: I812f308ef38833e89dfa2ce01263a297032f563a
Closes-Bug: #1273993

heat/common/environment_format.py

@@ -24,9 +24,10 @@ SECTIONS = (PARAMETERS, RESOURCE_REGISTRY) = \
 def parse(env_str):
     '''
     Takes a string and returns a dict containing the parsed structure.
-    This includes determination of whether the string is using the
-    JSON or YAML format.
     '''
+    if env_str is None:
+        return {}
+
     try:
         env = yaml.load(env_str, Loader=yaml_loader)
     except yaml.YAMLError as yea:

heat/tests/test_environment_format.py

@@ -45,6 +45,9 @@ parameters: }
 '''
         self.assertRaises(ValueError, environment_format.parse, env)
 
+    def test_yaml_none(self):
+        self.assertEqual({}, environment_format.parse(None))
+
 
 class YamlParseExceptions(common.HeatTestCase):