[FIX] base: prevent tracebacks on rate creation

Since this commit, https://github.com/odoo/odoo/commit/32ada4a415a14052d780f351edb82a0011561451,
when adding a new rate from the tree view,
if the `name` field is emptied (despite having a default value),
the `_onchange_rate_warning()` method will generate a traceback
because it will call `_get_latest_rate()` that relies on the `name`
field to order the company rates.

Steps to reproduce:
- Go to the Accounting App.
- Click on Configuration > Currencies.
- Select any currency and click on "Add a line" in the Rates tab.
- Remove the default value from the "Date" field and leave it empty.
- Now try to change "Unit per USD". A traceback should be triggered.

New behavior:
With this fix, an error message will now invite the user to set the `name`
(a.k.a. 'Date') correctly.

closes odoo/odoo#100874

X-original-commit: ddced61ca27f96b89af32f0b99e225321f966681
Signed-off-by: William André (wan) <wan@odoo.com>
Signed-off-by: John Laterre (jol) <jol@odoo.com>

odoo/addons/base/models/res_currency.py

@@ -3,11 +3,11 @@
 
 import logging
 import math
-import re
-import time
 
 from lxml import etree
+
 from odoo import api, fields, models, tools, _
+from odoo.exceptions import UserError
 from odoo.tools import parse_date
 
 _logger = logging.getLogger(__name__)
@@ -349,6 +349,9 @@ class CurrencyRate(models.Model):
         return super().create([self._sanitize_vals(vals) for vals in vals_list])
 
     def _get_latest_rate(self):
+        # Make sure 'name' is defined when creating a new rate.
+        if not self.name:
+            raise UserError(_("The date for the current rate is empty.\nPlease set it."))
         return self.currency_id.rate_ids.sudo().filtered(lambda x: (
             x.rate
             and x.company_id == (self.company_id or self.env.company)