fix(support-bundle): default in-cluster collectors in host support bundle

[banjoh]

Description, Motivation and Context

Ensure cluster-resources and cluster-info collectors are present only when a support bundle spec contains in-cluster collectors.

If there is not running cluster, support-bundle logs an error like below which leads users to think that the support bundle was not generated. A follow-up PR will be created to fix that message.

[evans] $ support-bundle examples/collect/host/cpu.yaml

Error: failed to run collect and analyze process: failed to run collectors: failed to check RBAC for collectors: failed to run subject review: Post "http://localhost:8080/apis/authorization.k8s.io/v1/selfsubjectaccessreviews": dial tcp [::1]:8080: connect: connection refused
✘-1 ~/repos/replicated/troubleshoot [main {origin/main}|✚ 4…9⚑ 7]

Fixes: #1393

Checklist

• New and existing tests pass locally with introduced changes.
• Tests for the changes have been added (for bug fixes / features)
• The commit message(s) are informative and highlight any breaking changes
• Any documentation required has been added/updated. For changes to https://troubleshoot.sh/ create a PR here

Does this PR introduce a breaking change?

• Yes
• No

[xavpaice]

not sure why this is failing tests, but it is?

[banjoh]

not sure why this is failing tests, but it is?

Its a valid issue. I've fixed it in this commit. Essentially, a bug surfaced itself when collecting a support bundle using a spec like below

apiVersion: troubleshoot.sh/v1beta2
kind: Collector
metadata:
  name: collector-sample
spec:
  collectors: []

Golang treats empty slices the same as nil slices when it comes to its built in functions such as append(), len() etc. That last commit handles that peculiar behaviour.

[banjoh]

@xavpaice tests have been fixed