gh-153854: Raise ValueError from imaplib.Time2Internaldate on empty string

[fedonman]

imaplib.Time2Internaldate is documented to raise ValueError for input
that is not of a known type, and its final branch does so. However, the
string branch subscripted the value before that check:

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

so an empty string escaped as a bare IndexError:

>>> 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

This replaces the subscript comparison with str.startswith('"') and
str.endswith('"'), which handle the empty string gracefully while
preserving behaviour for every other input (a properly double-quoted
string is still passed through unchanged; anything else raises
ValueError).

Regression tests are added for the empty/malformed string cases and for
the quoted-string passthrough.

• Issue: imaplib.Time2Internaldate raises IndexError instead of ValueError on an empty string #153854

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[python-cla-bot]

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