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