Allow environment to not have resource_registry

If you try to use an environment file that contains only parameters,
or even nothing, you get a NoneType error as the client is expecting
there to be a value in the resource_registry section. This stops that
error, thus allowing environment files to be empty or just contain
parameters.

Change-Id: I0c6c3bb60182915df346ebf8dd4e98ccc652ffb1
Closes-Bug: #1273881