[FIX] web: reports, fallback for external report layout.

Following revision 61eef73b

the fallback to the standard external layout when
the external layout is not set on the company has been removed.

It therefore leaded to the raise of the exception
```
The report's template '%s' is wrong, please contact your administrator.
Can not separate file to save as attachment because the report's template
does not contains the attributes 'data-oe-model' and 'data-oe-id'
on the div with 'article' classname.") %  self.name)
```

when the external layout was not set,
which could happen quite easily in a multi-company environment,
if you do not set the company external layout after having created it.

This revision simply puts back the fallback as it was before.

opw-1931195

closes odoo/odoo#30687