[FIX] base: default signature

Set a default email signature to an empty string to avoid `False` when
evaluating `${user.signature | safe}` in email templates.

opw-2229846

closes odoo/odoo#49558

X-original-commit: 96f46fba2635d8050aef03c7d4744a87ac8a597e
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

odoo/addons/base/models/res_users.py

@@ -237,7 +237,7 @@ class Users(models.Model):
         help="Specify a value only when creating a user or if you're "\
              "changing the user's password, otherwise leave empty. After "\
              "a change of password, the user has to login again.")
-    signature = fields.Html(string="Email Signature")
+    signature = fields.Html(string="Email Signature", default="")
     active = fields.Boolean(default=True)
     active_partner = fields.Boolean(related='partner_id.active', readonly=True, string="Partner is Active")
     action_id = fields.Many2one('ir.actions.actions', string='Home Action',