FIx NPE in InstanceMetricAlarmAssertWorker (#1614)

* add user

* add user

* FIx NPE in InstanceMetricAlarmAssertWorker

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

@@ -66,7 +66,7 @@ public class InstanceMetricAlarmAssertWorker extends AlarmAssertWorker<InstanceM
         Instance instance = instanceDAO.getInstance(alarm.getInstanceId());
         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();
         }