doors: Allow loginbroker property to be empty

It should be possible to not register with any loginbroker.

Target: trunk
Require-notes: no
Require-book: no
Request: 2.7
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6219/
(cherry picked from commit eb26c213d4ae019eca7bed9c02d30c19c746b1bd)
Merge pull request #270 from gbehrmann/fix/2.7/rb6219

doors: Allow loginbroker property to be empty