fix(querier): fixes panic when filtering agg metrics from nil resp

[ashwanthgoli]

What this PR does / why we need it:

/src/enterprise-logs/vendor/github.com/grafana/loki/v3/pkg/querier/handler.go:58 +0xb55 |  
github.com/grafana/loki/v3/pkg/querier.(*Handler).Do(0xc0028986d8, {0x555c0a8, 0xc000aa7a40}, {0x558c8e0, 0xc0008018f0}) |  
/src/enterprise-logs/vendor/github.com/grafana/loki/v3/pkg/querier/http.go:203 +0x569 |  
github.com/grafana/loki/v3/pkg/querier.(*QuerierAPI).SeriesHandler(_, {_, _}, _) |  
/src/enterprise-logs/vendor/github.com/grafana/loki/v3/pkg/querier/http.go:341 +0x2c |  
github.com/grafana/loki/v3/pkg/querier.(*QuerierAPI).filterAggregatedMetricsFromSeriesResp(0xc0028be370?, 0x0) |  
/usr/local/go/src/runtime/panic.go:792 +0x132

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.
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[sandeepsukhani]

Not going to block the PR on this, but I think we should return early here with nil check instead of doing the nil check at multiple places. Even better if we know resp would be nil only in case of err, and put an error check above at line 186 or 187 and return early.

[ashwanthgoli]

we might not be able to return early since the stats reporting happens for failed requests too. I moved this up though to avoid checks at multiple places

[sandeepsukhani]

approved it with a non-blocking comment