doors: Fix parsing of cipher flags

Addresses the problem that the empty string was not accepted for
*.security.ciphers configuraiton properties.

Target: trunk
Request: 2.6
Request: 2.2-sha2
Require-book: no
Require-notes: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5795/