Can't update start user id for historic process instance

**Describe the bug**
Can't update start user id for historic process instance.
I checked the mybatis mapping xml for historicProcessInstance, there is no update sql for start user id. We need to update the start user id to null when the user need to be off boarding.

**Expected behavior**
The start user id can be updated.

**Code**
HistoricProcessInstanceEntityManager historicProcessInstanceEntityManager =
CommandContextUtil.getHistoricProcessInstanceEntityManager();
List processInstanceList =
historicProcessInstanceEntityManager.findHistoricProcessInstancesByQueryCriteria(historicProcessInstanceQuery);
for (HistoricProcessInstance processInstance : processInstanceList) {
HistoricProcessInstanceEntity processEntity = (HistoricProcessInstanceEntity) processInstance;
processEntity.setStartUserId(null);
historicProcessInstanceEntityManager.update(processEntity);
}

**Additional context**
Flowable version: 6.4.2
Database: postgresql 11

Comienza con el XML de mapeo de MyBatis de HistoricProcessInstance y la ruta de actualización de HistoricProcessInstanceEntityManager mencionada en el issue. Reproduce la actualización con startUserId establecido en null en PostgreSQL y verifica después que la instancia histórica del proceso persiste el valor borrado.

Escrito por el modelo de indexación a partir del texto del issue.