Fix log dumping for GKE

[tosi3k]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Recent log-dump.sh change in #94762 has broken log dumping in GKE. According to the build log the log dumping process fails here because master_names variable is unbound. I traced the change and found out that the relevant part of bash was changed from

local master_names
if [[ -n "${use_custom_instance_list}" ]]; then
    master_names=( $(log_dump_custom_get_instances master)
(...)
fi
(...)
if [[ "${#master_names[@]}" == 0 ]]; then
  echo "No masters found?"
  return
fi

to

local master_names
if [[ -n "${use_custom_instance_list}" ]]; then
  while IFS='' read -r line; do master_names+=("$line"); done < <(log_dump_custom_get_instances master)
(...)
fi
if [[ "${#master_names[@]}" == 0 ]]; then
  echo 'No masters found?'
  return
fi

Because log_dump_custom_get_instances doesn't echo anything for GKE, we end up with unbound master_names variable. Initializing it to the empty array () resolves this problem.

Which issue(s) this PR fixes:
No issue is opened for that.

Special notes for your reviewer:
/sig scalability
/assign @wojtek-t @jkaniuk

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

@tosi3k: This issue is currently awaiting triage.

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[tosi3k]

/test pull-kubernetes-bazel-test

[jkaniuk]

/lgtm

[wojtek-t]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tosi3k, wojtek-t

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• cluster/log-dump/OWNERS [wojtek-t]

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[tosi3k]

/test pull-kubernetes-node-e2e

[tosi3k]

/retest