Fix invalid message from gnt-os

When we still don't have any os scripts installed, running gnt-os
diagnose or gnt-os info foo(whatever) shows an error message "Can't get
the OS list" even if the request has been succeeded.
The OpOsDiagnose can return a blank list, so we should check whether
the result is None.
Signed-off-by: Yuto KAWAMURA(kawamuray) <kawamuray.dadada@gmail.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>

lib/client/gnt_os.py

@@ -88,7 +88,7 @@ def ShowOSInfo(opts, args):
                             names=[])
   result = SubmitOpCode(op, opts=opts)
 
-  if not result:
+  if result is None:
     ToStderr("Can't get the OS list")
     return 1
 
@@ -156,7 +156,7 @@ def DiagnoseOS(opts, args):
                                            "blacklisted"], names=[])
   result = SubmitOpCode(op, opts=opts)
 
-  if not result:
+  if result is None:
     ToStderr("Can't get the OS list")
     return 1