Tolerate an empty environment properties.

An environment file containing the following:
properties:

Results in the following error:
 File "/home/steveb/dev/localstack/heat/heat/api/openstack/v1/stacks.py",
line 132, in environment
    env[self.PARAM_USER_PARAMS].update(parameters)
AttributeError: 'NoneType' object has no attribute 'update'

Change-Id: I7b36d797383cc3c74d4d0c77c283564b01aa6a6b
Merge "Tolerate an empty environment properties."