Don't join None values in utils.urljoin

This was seen on resources that don't set an id attribute, ending up
with request URLs such as /v1/blargablarga/None instead of
/v1/blargablarga/.

Change-Id: Ib6fa52c6d4b76f6c9d30418277fdd927afb5d102
Merge "Don't join None values in utils.urljoin"