Ignore synced config zones where no config item exists

Ignore synced config zones where no config item exists

The culprit is that we're in compiling configuration stage here,
we don't have access to `Zone::GetByName()` as objects have not
been activated yet.

Our best guess is from a config item loaded before (e.g. from zones.conf)
since no-one can sync zones via cluster config sync either.

It may not be 100% correct since the zone object itself may be invalid.
Still, if the zone object validator fails later, the config breaks either way.

The problem with removal of these directories is dealt by the cluster
config sync with stages.

refs #6727
refs #6716