Do not crash for queries where no metrics are returned

Depending on the configured query and particular timespan, it is
possible for no metrics to be returned. Which in the previous
implementation would cause a panic. This commit gracefully handles the
scenario by continuing to the next query job in this particular case.

main.go

@@ -179,6 +179,12 @@ func runAPIPolling(done chan error, url, token string, yamlConfig YamlConfig, re
 				return
 			}
 
+			// Handle cases where the metric may be missing for the given time range
+			if len(poll.Events) < 1 {
+				zap.L().Sugar().Debugf("No Events returned by query. Timespan: %v, MetricName: %s", job.Timespan, job.MetricName)
+				continue
+			}
+
 			var floatValue float64
 			for _, f := range supportedFunctions {
 				value, ok := poll.Events[0][f]