fix roster regression

return empty member list when AppGroup is not found
diff --git a/lib/whimsy/asf/ldap.rb b/lib/whimsy/asf/ldap.rb
index 8a3918d..909eaf2 100644
--- a/lib/whimsy/asf/ldap.rb
+++ b/lib/whimsy/asf/ldap.rb
@@ -809,11 +809,18 @@
     @base = 'ou=apps,ou=groups,dc=apache,dc=org'
 
     def dn
-      @dn ||= ASF.search_subtree(self.class.base, "cn=#{name}", 'dn').first.first
+      return @dn if @dn
+      dns = ASF.search_subtree(self.class.base, "cn=#{name}", 'dn')
+      @dn = dns.first.first unless dns.empty?
+      @dn
     end
 
     def base
-      @base = dn.sub(/^cn=.*?,/, '')
+      if dn
+        dn.sub(/^cn=.*?,/, '')
+      else
+        super
+      end
     end
 
     def self.list(filter='cn=*')