Do not empty a sessions table the site may not have

One condition. yoyaku:load-clear --truncate empties three tables so an arm of a load run starts on an empty hall; two are this module's own and always exist, and the third is core's sessions, which a site keeping its sessions anywhere else does not have.

Emptying a table that is not there ends the reset, and the reset is the first thing every arm does, so what is lost is the whole run rather than one figure. Hit while running the harness against a freshly built rig.

No test, deliberately. The branch a test would need is "the table is absent", which a kernel test installs by default; making it absent would mean dropping a core table underneath a running site, which proves nothing about this module. The guard reads as its own proof.