fix: Fix panic in ingester.go

[grobinson-grafana]

What this PR does / why we need it:

Fixes a panic in ingester.go because the slice is nil.

func (i *Ingester) InitFlushWorkers() {
	i.flushWorkersDone.Add(i.cfg.ConcurrentFlushes)
	for j := 0; j < i.cfg.ConcurrentFlushes; j++ {
		i.flushBuffers[j] = new(bytes.Buffer)
		go i.flushWorker(j)
	}
}

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[cyriltovena]

LGTM