Allow empty response in service-list

If a user runs 'nova service-list --binary something-weird', where
something-weird is a binary that is not listed the result is a confusing
IndexError. That is because the result is expected to always have at
least one result.

Part of the reason for that is that the fake client does not make it
easy to simulate an empty response in tests. That is why there is no
regression test associated with this bug-fix. That is also why I am
marking thas as Partial-Bug, as this corrects the symptom, but it does
not prevent it from regressing.

Change-Id: I3e6798d63a662c245196b3bf95bb7e1b0ea9c3a4
Partial-Bug: #1260504
Merge "Allow empty response in service-list"