Modify empty and default cases for workflow configuration

Fix several inconsistencies with the treatment of status_enum_workflow
configuration.

1. Fix showing a non existant status row in workflow config array. This
   is interpreted as if that status allows all transitions, but in the
   config page, the checks were shown unmarked, which can be confusing.
   Now the checks appear marked in this situation.

2. As consequence of (1), the message "You cannot move an issue out
   of this status" is not show anymore in that situation.

3. When a status row is saved with all transitions unchecked, an empty
   enum string is saved, meaning effectively that no transitions are
   allowed from that status. This specific configuration was previously
   unavailable from the workflow config page.

Fixes 0020682

Signed-off-by: Damien Regad dregad@mantisbt.org

Original commit message reworded and formatted