Fix RequestContext crashes w/ no service catalog

Code introduced in a prev review assumed service_catalog will be
at least an empty list and will never be None.  But missed one code
path where the service_catalog could potentially be set to None. So
this change switches back the default value of service_catalog in
the arguments to None and makes sure we tolerate anyone else
passing in a None or Empty list as service_catalog

Fix for LP# 1156730

Change-Id: I480b761d57c4699ea7ef72114160cdbeb281e454
Merge "Fix RequestContext crashes w/ no service catalog"