Fall back to 'en' if format_date is called with a falsy value

Refs python-babel/babel#1183

Author: akx

sphinx/util/i18n.py

@@ -228,6 +228,14 @@ def babel_format_date(
     if not hasattr(date, 'tzinfo'):
         formatter = babel.dates.format_date
 
+    if not locale:
+        # Babel would not accept a falsy locale
+        # (or would try to fall back to the LC_TIME
+        # locale, which would be not what was requested),
+        # so we can just short-cut to English, as we
+        # would for the `"fallback to English"` case.
+        locale = 'en'
+
     try:
         return formatter(date, format, locale=locale)
     except (ValueError, babel.core.UnknownLocaleError):

tests/test_util/test_util_i18n.py

@@ -4,12 +4,10 @@
 
 import datetime
 import os
-import sys
 import time
 from pathlib import Path
 from typing import TYPE_CHECKING
 
-import babel
 import pytest
 from babel.messages.mofile import read_mo
 
@@ -60,11 +58,6 @@ def test_catalog_write_mo(tmp_path):
         assert read_mo(f) is not None
 
 
-# https://github.com/python-babel/babel/issues/1183
-@pytest.mark.xfail(
-    sys.platform == 'win32' and babel.__version__ == '2.17.0',
-    reason='Windows tests fail with Babel 2.17',
-)
 def test_format_date():
     date = datetime.date(2016, 2, 7)