If the result of an opcode was a non-empty dictionary, it
would be impossible to differenciate between input and result:

  Input fields:
    […]
    debug_level: 0
    fields: cluster_name,master_node,volume_group_name
    jobs: [[True, u'37922'], [True, u'37923'], [True, u'37924']]

Expected output:

  Input fields:
    […]
    debug_level: 0
    fields: cluster_name,master_node,volume_group_name
  Result:
    jobs: [[True, u'37922'], [True, u'37923'], [True, u'37924']]

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