fix slice error when fetching network metrics (highlight#7227)

## Summary

Fixes runtime error
https://app.highlight.io/1/errors/h2yfoNmOw44yHVUc1uzdeDCFba8Y/instances/226248307

## How did you test this change?

Validating that the error does not happen when locally loading a network
request in dev tools.

## Are there any deployment considerations?

No

## Does this work require review from our design team?

No

backend/private-graph/graph/schema.resolvers.go

@@ -7130,10 +7130,12 @@ func (r *queryResolver) NetworkHistogram(ctx context.Context, projectID int, par
 			Count:    int(count),
 		})
 	}
-	sort.Slice(result.Buckets, func(i, j int) bool {
-		return result.Buckets[i].Count > result.Buckets[j].Count
-	})
-	result.Buckets = result.Buckets[0:lo.Min([]int{30, len(result.Buckets) - 1})]
+	if len(result.Buckets) > 0 {
+		sort.Slice(result.Buckets, func(i, j int) bool {
+			return result.Buckets[i].Count > result.Buckets[j].Count
+		})
+		result.Buckets = result.Buckets[0:lo.Min([]int{30, len(result.Buckets) - 1})]
+	}
 
 	return result, nil
 }