fix panic when networkTraffic is nil

Signed-off-by: dev <dev@localhost>

Author: dev

pkg/util/execdetails/tiflash_stats.go

@@ -718,6 +718,9 @@ func (networkTraffic *TiFlashNetworkTrafficSummary) mergeExecSummary(summary *ti
 // GetInterZoneTrafficBytes returns the inter zone network traffic bytes involved
 // between tiflash instances.
 func (networkTraffic *TiFlashNetworkTrafficSummary) GetInterZoneTrafficBytes() uint64 {
+	if networkTraffic == nil {
+		return 0
+	}
 	// NOTE: we only count the inter zone sent bytes here because tiflash count the traffic bytes
 	// of all sub request. For each sub request, both side with count the send and recv traffic.
 	// So here, we only use the send bytes as the overall traffic to avoid count the traffic twice.