Fix nil handling in NodeGroupForNode wrapper

[omerap12]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Improves checking for empty cloudprovider.NodeGroup results. Uses reflect.ValueOf().IsNil() to catch both nil interfaces and interfaces holding nil values.

Which issue(s) this PR fixes:

Fixes #7209

Special notes for your reviewer:

Does this PR introduce a user-facing change?

None

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

None

[dbonfigli]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dbonfigli, omerap12

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• cluster-autoscaler/cloudprovider/externalgrpc/OWNERS [dbonfigli]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[Shubham82]

I think we should Backport these changes to the supported previous releases so that the bug will be fixed in their upcoming patch release (#7315)

If it seems good to you then I will raise PRs for it.

[Shubham82]

cc @dbonfigli @omerap12
WDYT?

[dbonfigli]

The fix was about an example, so not sure if that is so important.

[Shubham82]

@dbonfigli, I thought wrapper.go is part of the externalgrpc cloud provider code, but if it is part of the example there is no need to backport.

Thanks for the information.