fix: panics when ingester response is nil

[shantanualsi]

What this PR does / why we need it:

This PR fixes panics when ingester response is nil

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
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[benclive]

Do we know why the ingester responses could be nil? This seems to fix the panics but I'm curious how we're getting nil responses in the first place

[MichelHollands]

LGTM

[shantanualsi]

Do we know why the ingester responses could be nil?
@benclive my guess is because the time range is older than query_ingesters_within (which is default 3h).. In this case, the ingesters do not have any labels..