[Fix-14895][API] Keep printing null if task log file not exist (#14912)

dolphinscheduler-api/src/main/java/org/apache/dolphinscheduler/api/service/impl/LoggerServiceImpl.java

@@ -261,7 +261,9 @@ public class LoggerServiceImpl extends BaseServiceImpl implements LoggerService
                 log.error("Error while getting log from remote target", e);
             }
         }
-        sb.append(logContent);
+        if (logContent != null) {
+            sb.append(logContent);
+        }
         return sb.toString();
     }