[FIX] web: avatar on empty field

- Create a calendar event
- In debug mode, remove the responsible

The 'Avatar Undefined' string is displayed at the botton of the
responsibles' list.

Do no fetch an image for a falsy value (= falsy id)

opw-2010007

closes odoo/odoo#34215

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

addons/web/static/src/xml/web_calendar.xml

@@ -52,7 +52,7 @@
                             <t t-if="filter.value == 'all'">
                                 <span><i class="fa fa-users fa-fw o_cal_avatar" role="img" aria-label="Avatar" title="Avatar"/></span>
                             </t>
-                            <t t-if="widget.avatar_field &amp;&amp; (filter.value != 'all')">
+                            <t t-if="widget.avatar_field &amp;&amp; (filter.value != 'all') &amp;&amp; (filter.value)">
                                 <img t-attf-src="/web/image/#{widget.avatar_model}/#{filter.value}/#{widget.avatar_field}" class="o_cal_avatar" alt="Avatar"/>
                             </t>
                             <t t-set="color" t-value="widget.getColor(filter.color_index)"/>