Hello.
Since man-db is happy to accept an empty file at sysconfdir/man_db.conf and it works fine with this empty file on systems that use the defaults, it seems like it would be okay to accept the file not existing at all.
Currently, if the file doesn't exist this happens:
$ man man
man: can't open the manpath configuration file /etc/man_db.conf`
Removing this error message would make it simpler for packaging distributions who use the defaults to package man-db.
Ryan