[FIX] mass_mailing: do not crash when importing a void contact list

Mass mailing has a wizard to import contacts from a text input. It currently
crashes when trying to import a void field, while it should not.

We fix that by two means
  * make field required in view (as model cannot be modified);
  * while it is not updated, simply consider a void input should not crash and
    is considered as an input without valid input found;

Task-3053031

closes odoo/odoo#104769

X-original-commit: aa183fa29d609d2b608fa451ee48a35e3d1549b2
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mass_mailing/wizard/mailing_contact_import.py

@@ -15,8 +15,7 @@ class MailingContactImport(models.TransientModel):
     def action_import(self):
         """Import each lines of "contact_list" as a new contact."""
         self.ensure_one()
-
-        contacts = tools.email_split_tuples(', '.join(self.contact_list.splitlines()))
+        contacts = tools.email_split_tuples(', '.join((self.contact_list or '').splitlines()))
         if not contacts:
             return {
                 'type': 'ir.actions.client',

addons/mass_mailing/wizard/mailing_contact_import_views.xml

@@ -17,6 +17,7 @@
                 <label for="contact_list" class="mb-2">Contact List</label>
                 <field name="contact_list" class="o_mailing_contact_import_list"
                     nolabel="1" default_focus="1"
+                    required="True"
                     placeholder='"Damien Roberts" &lt;d.roberts@example.com&gt;&#10;"Rick Sanchez" &lt;rick_sanchez@example.com&gt;&#10;victor_hugo@example.com'/>
                 <p class="text-muted mb-0">
                     Want to import country, company name and more?