[DS-6502][MasterServer] fix cmdParam NPE when constructProcessInstance on SCHEDULER mode (#6504)

Co-authored-by: caishunfeng <534328519@qq.com>

dolphinscheduler-service/src/main/java/org/apache/dolphinscheduler/service/process/ProcessService.java

@@ -792,7 +792,7 @@ public class ProcessService {
             }
         }
         // reset command parameter if sub process
-        if (cmdParam.containsKey(Constants.CMD_PARAM_SUB_PROCESS)) {
+        if (cmdParam != null && cmdParam.containsKey(Constants.CMD_PARAM_SUB_PROCESS)) {
             processInstance.setCommandParam(command.getCommandParam());
         }
         if (Boolean.FALSE.equals(checkCmdParam(command, cmdParam))) {