Fix NPE in alarm content generation. (#1558)

* Update InstanceReferenceMetricAlarmAssertWorker.java

增加osInfo 判空

* Update InstanceReferenceMetricAlarmAssertWorker.java

增加osinfo 判空

apm-collector/apm-collector-analysis/analysis-alarm/alarm-provider/src/main/java/org/apache/skywalking/apm/collector/analysis/alarm/provider/worker/instance/InstanceReferenceMetricAlarmAssertWorker.java

@@ -68,7 +68,7 @@ public class InstanceReferenceMetricAlarmAssertWorker extends AlarmAssertWorker<
         Instance instance = instanceDAO.getInstance(alarm.getBehindInstanceId());
         JsonObject osInfo = gson.fromJson(instance.getOsInfo(), JsonObject.class);
         String serverName = Const.UNKNOWN;
-        if (osInfo.has("hostName")) {
+        if (osInfo != null && osInfo.has("hostName")) {
             serverName = osInfo.get("hostName").getAsString();
         }