imaplib.Time2Internaldate raises IndexError instead of ValueError on an empty string

Bug report

Bug description:

imaplib.Time2Internaldate is documented to raise ValueError for input that is not of a known type, and its own final branch is raise ValueError("date_time not of a known type"). However, the string branch subscripts the value before that check:

elif isinstance(date_time, str) and (date_time[0],date_time[-1]) == ('"','"'):

so an empty string escapes as a bare IndexError instead:

>>> import imaplib
>>> imaplib.Time2Internaldate('')
Traceback (most recent call last):
  ...
IndexError: string index out of range
>>> imaplib.Time2Internaldate('x')  # non-empty unquoted string is fine
Traceback (most recent call last):
  ...
ValueError: date_time not of a known type

The fix is to use slices so the comparison is simply false for an empty string and control falls through to the intended ValueError:

elif isinstance(date_time, str) and (date_time[:1], date_time[-1:]) == ('"', '"'):

Behavior for all other inputs is unchanged. Related: gh-86165 recently fixed a different crash in this same function. I have a PR ready with the fix, a regression test, and a NEWS entry.

CPython versions tested on:

3.12, CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-153874
• gh-153929

エントリポイント imaplib.Time2Internaldate から開始し、issue で報告されている空文字列のパスを確認します。issue で言及されている回帰テストと NEWS エントリを確認します。空の入力で ValueError が発生し、その他の入力では動作が維持されれば完了です。