gh-161222: Fix email.utils.encode_rfc2231() writing "None" as the charset

Lib/email/utils.py

@@ -394,6 +394,8 @@ def encode_rfc2231(s, charset=None, language=None):
     s = urllib.parse.quote(s, safe='', encoding=charset or 'ascii')
     if charset is None and language is None:
         return s
+    if charset is None:
+        charset = ''
     if language is None:
         language = ''
     return "%s'%s'%s" % (charset, language, s)

Lib/test/test_email/test_email.py

@@ -5926,6 +5926,16 @@ def test_rfc2231_unencoded_then_encoded_segments(self):
         eq(language, 'en-us')
         eq(s, 'My Document For You')
 
+    def test_rfc2231_encode_language_without_charset(self):
+        eq = self.assertEqual
+        eq(utils.encode_rfc2231('abc', language='en'), "'en'abc")
+        msg = Message()
+        msg.add_header('Content-Disposition', 'attachment',
+                       filename=(None, 'en', 'report.txt'))
+        eq(msg['Content-Disposition'],
+           "attachment; filename*='en'report.txt")
+        eq(msg.get_filename(), 'report.txt')
+
     def test_should_not_hang_on_invalid_ew_messages(self):
         messages = ["""From: user@host.com
 To: user@host.com

Misc/NEWS.d/next/Library/2026-11-02-20-08-49.gh-issue-161222.e4eARb.rst

@@ -0,0 +1,3 @@
+Fix :func:`email.utils.encode_rfc2231` writing the literal string ``None``
+as the charset when only *language* is given.  The charset field is now left
+blank, as permitted by :rfc:`2231`.