After (manual) upgrade from 11.2-U8 to 12.0-U2.1 (with an intermediate upgrade to 11.3-U5), /usr/local/etc/collectd.conf is empty (0 bytes).
I was debugging why graphite reporting was not working, which led me to realize that /var/db/collectd/rrd does not exist. Then found out that collectd did not have the right configuration, and then realized that collectd.conf is empty.
Machine is dual Xeon 6140M Intel system with 256GB of RAM and 12 spindle SAS disks.
I have a second TrueNAS 12.0-U2.1 system (VM) that has no issues.