aaa: Fix npe in search when authz is not properly configured

Topic: AAA
NPE occured as the namespaces list returned null in case
authz extension is not configured properly.

Change-Id: Ic7b7c196b01f1572d6013996303f26ccd3d782d0
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1158512
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>