Fixes catalog URL formatting to never return None

The old behavior was to return None if the URL template was not a string
(or not string-like). This was a mistake because a None should never be
able to make its way into the catalog.

This is based off of an IRC discussion that spawned from:
https://review.openstack.org/#/c/81528/4/keystone/catalog/backends/sql.py

Change-Id: I12b0362d3869a3ec8dc1a6fa34e934a221deecbc
Partial-bug: #1230279
Merge "Fixes catalog URL formatting to never return None"