Fix nil pointer deref in metrics exporter (#1187)

[zcross]

This is a very targeted fix for the NPE shown in #1187 since it's a known problem. The general pattern of guarding status lookups may be worth thinking about. Maybe we just need an intermediate type that decouples the application level usage of this state from its serialization/deserialization, at which point we can use basic language features like field visibility to reduce the probability of future unsafe accesses.

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[zcross]

Meta: it would seem to be going against the trend to add some highly specific unit test for this, so I'm going to let integration tests "cover it" ... although they didn't catch its first introduction (possibly due to the self-healing / temporal element of the "status" state). I made this quick edit in a text editor so I'm crossing my fingers for a green build :)

[zcross]

Whoops:

133.0 pkg/apis/metrics/exporter.go:295:8: invalid operation: operator ! not defined on chi.EnsureStatus().GetNormalizedCHICompleted() (value of type *"github.com/***/clickhouse-operator/pkg/apis/clickhouse.***.com/v1".ClickHouseInstallation)

I'll fire this up on my actual development machine and fix that now