workflows: task_counter value check

* Checks if the task_counter parameter is a list when saving
  an BibWorkflowObject. It is required to avoid a problem with the
  underlying workflow engine.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
workflows: Holding Pen details fix

* Fixes a problem where the detailed page for Holding Pen
  object caused an exception when information on next/previous
  objects from the table were missing.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>