pythonGH-27946)Various date parsing utilities in the email module, such asemail.utils.parsedate(), are supposed to gracefully handle invalidinput, typically by raising an appropriate exception or by returningNone.The internal email._parseaddr._parsedate_tz() helper used by some ofthese date parsing routines tries to be robust against malformed input,but unfortunately it can still crash ungracefully when a non-empty butwhitespace-only input is passed. This manifests as an unexpectedIndexError.In practice, this can happen when parsing an email with only a newlineinside a ‘Date:’ header, which unfortunately happens occasionally in thereal world.Here's a minimal example:    $ python    Python 3.9.6 (default, Jun 30 2021, 10:22:16)    [GCC 11.1.0] on linux    Type "help", "copyright", "credits" or "license" for more information.    >>> import email.utils    >>> email.utils.parsedate('foo')    >>> email.utils.parsedate(' ')    Traceback (most recent call last):      File "<stdin>", line 1, in <module>      File "/usr/lib/python3.9/email/_parseaddr.py", line 176, in parsedate        t = parsedate_tz(data)      File "/usr/lib/python3.9/email/_parseaddr.py", line 50, in parsedate_tz        res = _parsedate_tz(data)      File "/usr/lib/python3.9/email/_parseaddr.py", line 72, in _parsedate_tz        if data[0].endswith(',') or data[0].lower() in _daynames:    IndexError: list index out of rangeThe fix is rather straight-forward: guard against empty lists, aftersplitting on whitespace, but before accessing the first element.(cherry picked from commit989f6a3)Co-authored-by: wouter bolsterlee <wouter@bolsterl.ee>