Make gnt-os list work with no OSes

When absolutely no OSes are present on the cluster, the result of
OpOsDiagnose is an empty list. This is currently handled in gnt-os as
an error condition, probably due to how OpOsDiagnose used to return
errors in the past.

However, with the query backend, an empty list is a reasonable answer,
so I see no reason why gnt-os should treat that as an error,
especially as it doesn't give any hint of what is "wrong". Simply
removing that code block allows it to proceed normally, and list no
OSes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>