[FIX] account: fallback on missing template

template contains the mail template to render and is the result of the call to
`self.env.ref('account.email_template_edi_invoice', False)`

If the template does not exists (deleted), template is `None` and the action
rendering crashes.
While it is not recommended to delete master data, it is still possible to use
custom mail templates.

Closes #15204

addons/account/account_invoice.py

@@ -395,7 +395,7 @@ class account_invoice(models.Model):
             default_model='account.invoice',
             default_res_id=self.id,
             default_use_template=bool(template),
-            default_template_id=template.id,
+            default_template_id=template and template.id or False,
             default_composition_mode='comment',
             mark_invoice_as_sent=True,
         )