`configparser.getboolean()` raises `AttributeError` for a value-less option

Bug description

When ConfigParser uses allow_no_value=True, calling getboolean() on a value-less option raises AttributeError instead of the documented ValueError.

Documentation:
https://docs.python.org/3/library/configparser.html#configparser.ConfigParser.getboolean

Reproducer

import configparser

config = configparser.ConfigParser(allow_no_value=True)
config.read_string("""
[section]
enabled
""")

assert config.get("section", "enabled") is None
config.getboolean("section", "enabled")

Current result:

AttributeError: 'NoneType' object has no attribute 'lower'

Expected result:

ValueError: Not a boolean: None

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-155437

从 ConfigParser.getboolean() 入口开始，重现 issue 中无值选项的情况。检查现有的 configparser 布尔转换测试，并添加覆盖范围以显示对 None 的文档化 ValueError；当此情况不再引发 AttributeError 时，工作就完成了。