[FIX] mass_mailing: allow upload a file when the contact list value is empty

Currently, in mass mailing, the user cannot be redirected to the next
page after clicking "upload a file" in the import mailing list.

This is because the contact list field is mandatory, so when the user
clicks "upload a file" with an empty contact list, it is unable to
be redirected to the next page, as it saves the wizard before executing the
action.

This commit fixes the above issue by making the contact list field
not mandatory anymore in the wizard view.

Oversight of: aa183fa29d609d2b608fa451ee48a35e3d1549b2

taks-3070829

closes odoo/odoo#106237

X-original-commit: 21272221ebf22b3075c79a759c8ad8f72825b1ef
Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>

addons/mass_mailing/wizard/mailing_contact_import_views.xml

@@ -17,7 +17,6 @@
                 <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?