fix(helm): make loki.storage.bucketNames are optional, if builtin minio is enabled.

[jkroepke]

What this PR does / why we need it:

If minio is enabled, the names of the bucket names are hard-coded and loki.storage.bucketNames is ignored.

loki/production/helm/loki/templates/_helpers.tpl

Lines 213 to 216 in e0b3353

	{{- if .Values.minio.enabled -}}
	s3:
	endpoint: {{ include "loki.minio" $ }}
	bucketnames: chunks

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

CI is currently failing: https://github.com/grafana/loki/actions/runs/16606809856/job/46981528703?pr=18510

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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