Create nil-safe wrapper around Items

[justinsb]

Avoids a potential panic.

[k8s-ci-robot]

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This pull-request has been approved by: justinsb

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[johnsonj]

@justinsb There's a few other usages of Items directly, consider making the value private and moving all callers over to GetItems

• https://github.com/kubernetes-sigs/kubebuilder-declarative-pattern/blob/beb06a5d4ddd3387c1aaa7694a587462da5c58e6/pkg/patterns/declarative/pkg/applier/applylib.go
• https://github.com/kubernetes-sigs/kubebuilder-declarative-pattern/blob/4ba4cadcd4ca182c7b29c0569c85fbbd2996c762/pkg/patterns/declarative/application.go

/lgtm