MessageTemplate API - Fix saving of templates with workflow_name sans workflow_id

MessageTemplate API - Fix saving of templates with workflow_name sans workflow_id

Overview
--------

This fixes a bug when saving (updating) a `MessageTemplate` record that
involves a `workflow_name` and no `workflow_id`. It is an alternative to #21674.

Steps to  reproduce
-------------------

Suppose you read a MessagTemplate with values:

```php
$values = ['workflow_name' => 'foo', 'workflow_id' => NULL, ...]
```

Then you save it back:

```php
civicrm_api4('MessageTemplate', 'update', [
  'values' => $values
]);
```

Before
------

The `update` raises an exception.

After
-----

The `update` works.