packet-netflow.c:  netflow v9: Allow options template with zero-length scope section

   Fixes Bug #10432

   Also: rework several comments.

Change-Id: I292829f6dffaf5f500cb089cc8a45e1203a2e731
Reviewed-on: https://code.wireshark.org/review/3959
Reviewed-by: Bill Meier <wmeier@newsguy.com>