gh-159621: Handle empty lists in TokenList.startswith_fws

[KohakuBlueleaf]

This PR fixed #159621

from email.parser import Parser
from email import policy

value = "a" * 50 + "@example.com,()b@example.com"
m = Parser(policy=policy.default).parsestr(f"To: {value}\r\n\r\nx")

assert m["To"].defects == ()
m.as_string()  # IndexError

The m.as_string() call startswith_fws in empty tokenlist cause IndexError

def startswith_fws(self):
   # return self[0].startswith_fws() <- if self is empty, this will error
   return self and self[0].startswith_fws()

As in some cases the startswith_fws may call within an empty tokenlist, I think check whether "self" is non empty is needed here.
In the same file the "WhiteSpaceTerminal" also do same "non empty check" before testing the self[0] in WPS

• Issue: email: IndexError when folding address headers containing empty comments #159621

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.