NetBackup should not skip a file restore simply because of it's size is zero - what's your NetBackup version? (but I don't think there is any bug in NetBackup since version 4.x days that is failing to restore zero-byte files...)
I wonder if the file was somehow deleted right after the restore?
sharetab will list all the volumes currently being shared, and I think it's related to NFS which might be dynamically updating the file.
Try touching another file in the same /etc/dfs directory, and then run backup and restore, also try restoring /etc/dfs directory and see if it's getting restored or not.
If you want to check out debug log, /usr/openv/netbackup/logs/tar will show the restore progress.
Abe