[FIX] mail: Give reply-to non-template value

When sending a mass mail through the composer, if the field ``reply_to`` had to
fall back to being ``email_from``, reply_to would take the value of the template
syntax instead of the rendered value.

This is notably the case when mass-mailing invoices through the accounting app.
Resulting in reply_to fields such as: '{{user.email}}'

On some mail clients (including mailhog), this could also result in template
syntax being shown as part of the subject or sender field.

This commit fixes that by correctly taking the rendered value of 'email_from'

Task-2816845

closes odoo/odoo#95227

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>