[utils] unified_strdate: Return None if the date format can't be recognized (fixes #7340)

This issue was introduced with ae12bc3ebb4cb377c2b4337ec255e652b36f5143, it returned 'None'.

test/test_utils.py

@@ -236,6 +236,7 @@ class TestUtil(unittest.TestCase):
             unified_strdate('2/2/2015 6:47:40 PM', day_first=False),
             '20150202')
         self.assertEqual(unified_strdate('25-09-2014'), '20140925')
+        self.assertEqual(unified_strdate('UNKNOWN DATE FORMAT'), None)
 
     def test_find_xpath_attr(self):
         testxml = '''<root>

youtube_dl/utils.py

@@ -911,7 +911,8 @@ def unified_strdate(date_str, day_first=True):
         timetuple = email.utils.parsedate_tz(date_str)
         if timetuple:
             upload_date = datetime.datetime(*timetuple[:6]).strftime('%Y%m%d')
-    return compat_str(upload_date)
+    if upload_date is not None:
+        return compat_str(upload_date)
 
 
 def determine_ext(url, default_ext='unknown_video'):