[FIX] mass_mailing, website_mass_mailing: empty section in settings

Install Email Marketing (mass_mailing) without Website.

From Email Marketing app, under Configuration -> Settings
the section "Mailing List" is empty.

This is not the expected behavior: any empty section should not be
visible.

To avoid breaking some third-party app which could rely on the element
'<div id="mailing_list" class="row mt16 o_settings_container"/>'
we choose to hide the title of the affected section, re-enabling it when
the appropriate app is installed.

opw-2045481
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closes odoo/odoo#35491

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/mass_mailing/views/res_config_settings_views.xml

@@ -58,7 +58,7 @@
                                 </div>
                             </div>
                         </div>
-                        <h2>Mailing Lists</h2>
+                        <h2 id="mailing_list_settings_title" invisible="1">Mailing Lists</h2>
                         <div id="mailing_list" class="row mt16 o_settings_container"/>
                     </div>
                 </xpath>

addons/website_mass_mailing/views/res_config_settings_views.xml

@@ -5,6 +5,9 @@
         <field name="name">res.config.settings.view.form.inherit.website.mass.mailing</field>
         <field name="model">res.config.settings</field>
         <field name="arch" type="xml">
+          <xpath expr="//h2[@id='mailing_list_settings_title']" position="attributes">
+            <attribute name="invisible">0</attribute>
+          </xpath>
             <xpath expr="//div[@id='mailing_list']" position="inside">
                 <div class="col-lg-6 o_setting_box col-12" title="This adds a new Newsletter Popup snippet to drag &amp; drop on your website pages. It triggers a popup when visitors land on the page. This popup prompts them to enter their email address to join a mailing list.">
                     <div class="o_setting_left_pane">