chk/controller: nil-check Get result in poll loop

[SAY-5]

Closes #1972. c.kube.CR().Get returns a nil interface on transient errors; the bare cur.(*apiChk.ClickHouseKeeperInstallation) then panics every reconcile.

[alex-zaitsev]

Duplicate of #1974

[sunsingerus]

Thanks for the contribution! Closing in favor of #1974, which addresses the same panic (issue #1972) with broader error handling — it follows the existing safe-cast pattern from pkg/controller/chk/kube/cr.go (statusUpdateProcess at lines 98-122): type-assert with ok, treat NotFound as terminal so polling stops cleanly when the CR is deleted, log+retry transient errors at V(1), and honor context cancellation before sleeping. #1974 has just been merged.