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

This issue was introduced with ae12bc3ebb, 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>