[FIX] mail: check all parameters are set

Since `model` is not a required field, the invalidation may crash when one is
missing.
It should never happen than a mail.message has a res_id but not a model as it
makes no business sence.
However it is possible than a message temporarly misses one of the two, e.g:

```
self.model = False
self.res_id = False
```
will trigger two writes and will crash at the first.

Above code should probably be refactored to have only one write but this commit
fixes a regression introduced at 8f1c2bfc (the above code did not crash).

Closes #15199

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