handle empty sections in environment files

If an environment file has an *empty* section, as in:

    parameter_defaults:

This would ultimately fail in deep_update (in
heatclient/common/template_utils.py), which would get called with old
== None.  This patch handles this situation by explicitly setting old
= {} if deep_update is called with old == None.

Change-Id: Ia7bee87cdb99e29c63c9dc163f8490fef3f24f24
Closes-bug: 1631408

heatclient/common/template_utils.py

@@ -188,6 +188,12 @@ def normalise_file_path_to_url(path):
 
 def deep_update(old, new):
     '''Merge nested dictionaries.'''
+
+    # Prevents an error if in a previous iteration
+    # old[k] = None but v[k] = {...},
+    if old is None:
+        old = {}
+
     for k, v in new.items():
         if isinstance(v, collections.Mapping):
             r = deep_update(old.get(k, {}), v)