[FIX] mail, *: add signature in email layouts only if requested and set

* = account, purchase, sale

Purpose of this commit is to add signature only if really asked. A variable
is available for that purpose (``email_add_signature``, recently renamed from
``add_sign``). We also correctly check signature is not void or pseudo-void
using tool ``is_html_empty``. Indeed editor may generates pseudo-void content
like ``<p><br /></p>``.

As signature is not added when a template is used (see composer code) we ensure
it is defined in mail templates used in "Send by email" flows. Indeed we cannot
know if a template already has a signature or not. To avoid having twice a
signature no signature is added in email notification layout when a template
is involved.

Continuation of odoo/odoo#76418 .

Task-2712450 (Mail/Sale: Improve 'Pay Now' notification template)

Part-of: odoo/odoo#82167

addons/account/data/mail_template_data.xml

@@ -46,7 +46,7 @@
         </t>
         <br /><br />
         Do not hesitate to contact us if you have any questions.
-        <t t-if="object.invoice_user_id.signature">
+        <t t-if="not is_html_empty(object.invoice_user_id.signature)">
             <br />
             <t t-out="object.invoice_user_id.signature or ''">--<br/>Mitchell Admin</t>
         </t>
@@ -75,7 +75,7 @@
         Do not hesitate to contact us if you have any questions.
         <br/><br/>
         Best regards,
-        <t t-if="user.signature">
+        <t t-if="not is_html_empty(user.signature)">
             <br/>
             <t t-out="user.signature or ''">--<br/>Mitchell Admin</t>
         </t>
@@ -119,7 +119,7 @@
         from <t t-out="object.company_id.name or ''">YourCompany</t>.
         <br /><br />
         Do not hesitate to contact us if you have any questions.
-        <t t-if="object.invoice_user_id.signature">
+        <t t-if="not is_html_empty(object.invoice_user_id.signature)">
             <br />
             <t t-out="object.invoice_user_id.signature or ''">--<br/>Mitchell Admin</t>
         </t>

addons/mail/data/mail_templates_email_layouts.xml

@@ -49,7 +49,7 @@
         <li><t t-esc="tracking[0]"/>: <t t-esc="tracking[1]"/> -&gt; <t t-esc="tracking[2]"/></li>
     </t>
 </ul>
-<div t-if="signature" t-out="signature" style="font-size: 13px;"/>
+<div t-if="email_add_signature and not is_html_empty(signature)" t-out="signature" style="font-size: 13px;"/>
 <p style="color: #555555; margin-top:32px;">
     Sent
     <span t-if="company.name">
@@ -177,12 +177,12 @@
                     <t t-esc="access_name"/>
                 </a>
             </div>
-            <t t-if="'user_id' in record and record.user_id and not record.env.user._is_superuser() and signature != ''">
+            <t t-if="email_add_signature and 'user_id' in record and record.user_id and not record.env.user._is_superuser() and not is_html_empty(record.user_id.sudo().signature)">
                 <div style="margin: 0px; padding: 0px; font-size:13px;">
                     Best regards,
                 </div>
                 <div>&amp;nbsp;</div>
-                <div t-if="record.user_id.sudo().signature" style="font-size: 13px;">
+                <div style="font-size: 13px;">
                     <div t-out="record.user_id.sudo().signature"/>
                 </div>
             </t>

addons/purchase/data/mail_template_data.xml

@@ -23,6 +23,10 @@
         If you have any questions, please do not hesitate to contact us.
         <br/><br/>
         Best regards,
+        <t t-if="not is_html_empty(object.user_id.signature)">
+            <br />
+            <t t-out="object.user_id.signature or ''">--<br/>Mitchell Admin</t>
+        </t>
     </p>
 </div></field>
             <field name="report_template" ref="report_purchase_quotation"/>
@@ -56,6 +60,11 @@
             <br/><br/>
             Could you please acknowledge the receipt of this order?
         </t>
+        <t t-if="not is_html_empty(object.user_id.signature)">
+            <br />
+            <t t-out="object.user_id.signature or ''">--<br/>Mitchell Admin</t>
+        </t>
+        <br/><br/>
     </p>
 </div></field>
             <field name="report_template" ref="action_report_purchase_order"/>
@@ -90,6 +99,11 @@
             <strong>undefined</strong>.
         </t>
         Could you please confirm it will be delivered on time?
+        <t t-if="not is_html_empty(object.user_id.signature)">
+            <br />
+            <t t-out="object.user_id.signature or ''">--<br/>Mitchell Admin</t>
+        </t>
+        <br/><br/>
     </p>
 </div></field>
             <field name="report_template" ref="action_report_purchase_order"/>

addons/sale/data/mail_template_data.xml

@@ -30,7 +30,11 @@
         </t>
         <br/><br/>
         Do not hesitate to contact us if you have any questions.
-        <br/>
+        <t t-if="not is_html_empty(object.user_id.signature)">
+            <br />
+            <t t-out="object.user_id.signature or ''">--<br/>Mitchell Admin</t>
+        </t>
+        <br/><br/>
     </p>
 </div>
             </field>
@@ -65,6 +69,10 @@
         </t>
         <br/><br/>
         Do not hesitate to contact us if you have any questions.
+        <t t-if="not is_html_empty(object.user_id.signature)">
+            <br />
+            <t t-out="object.user_id.signature or ''">--<br/>Mitchell Admin</t>
+        </t>
         <br/><br/>
     </p>
 <t t-if="hasattr(object, 'website_id') and object.website_id">