resolve NPE in getTemplateGroupStatistics method

Author: ericsson-peiwuyang

src/main/java/com/cisco/trex/stateful/TRexAstfClient.java

@@ -26,6 +26,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.Optional;
 import java.util.Set;
 import java.util.stream.Collectors;
 import java.util.stream.StreamSupport;
@@ -474,7 +475,7 @@ public Map<String, AstfStatistics> getTemplateGroupStatistics(
           try {
             AstfStatistics astfStatistics =
                 new ObjectMapper()
-                    .readValue(result.get(tgId.toString()).toString(), AstfStatistics.class);
+                    .readValue(Optional.ofNullable(result.get(tgId.toString())).map(t->t.toString()).orElse("{}"), AstfStatistics.class);
             astfStatistics.setCounterNames(metaData);
             stats.put(tgName, astfStatistics);
           } catch (IOException e) {