[FIX] mail: controller avatar return placeholder if no content

binary_content return '' if no picture, so, we keep the placeholder
if no image has been returned.

It avoid in the new portal with discuss widget to get the default
browser 404 picture (file broken)

This commit close #12683

addons/mail/controllers/main.py

@@ -222,7 +222,10 @@ class MailController(http.Controller):
                 # if the current user has access to the document, get the partner avatar as sudo()
                 request.env[res_model].browse(res_id).check_access_rule('read')
                 if partner_id in request.env[res_model].browse(res_id).sudo().exists().message_ids.mapped('author_id').ids:
-                    status, headers, content = binary_content(model='res.partner', id=partner_id, field='image_medium', default_mimetype='image/png', env=request.env(user=openerp.SUPERUSER_ID))
+                    status, headers, _content = binary_content(model='res.partner', id=partner_id, field='image_medium', default_mimetype='image/png', env=request.env(user=openerp.SUPERUSER_ID))
+                    # binary content return an empty string and not a placeholder if obj[field] is False
+                    if _content != '':
+                        content = _content
                     if status == 304:
                         return werkzeug.wrappers.Response(status=304)
             except AccessError: