roachtest: handle no ts datapoints in rebalance by-load roachtests

The `rebalance/by-load` roachtests collect timeseries data from the
cluster. If these tests started quickly enough, no datapoints could
exist which would cause an out of bounds array error.

Handle the no datapoints case, and log a message without failing the
test.

Informs: cockroachdb#107247

Release note: None

Author: kvoli

pkg/cmd/roachtest/tests/rebalance_load.go

@@ -304,6 +304,11 @@ func makeStoreCPUFn(
 		storesPerNode := numStores / numNodes
 		storeCPUs := make([]float64, numStores)
 		for node, result := range resp.Results {
+			if len(result.Datapoints) == 0 {
+				// If any node has no datapoints, there isn't much point looking at
+				// others because the comparison is useless.
+				return nil, errors.Newf("node %d has no CPU datapoints", node)
+			}
 			// Take the latest CPU data point only.
 			cpu := result.Datapoints[len(result.Datapoints)-1].Value
 			nodeIdx := node * storesPerNode