fix: dev: "rndc flushtree ." failed to flush the cache

`rndc flushtree` flushes cache data below a specified name. If the name
specified is the DNS root, it should fully empty the cache, the same
as `rndc flush`.  However, there was a bug causing the command, in that
case, to have no effect on the cache at all; this has been fixed.

Closes #6308

Merge branch '6308-rndc-flushtree-root' into 'main'

See merge request isc-projects/bind9!12579