gh-157466: Fix configparser writing a trailing space for empty values

[jang-hs]

RawConfigParser.write() builds the delimiter as " {} ".format(self._delimiters[0]) when space_around_delimiters is true, and _write_section() concatenates it unconditionally. An option with an empty value therefore gets only the delimiter, and the padding space is left dangling at the end of the line:

>>> c = configparser.ConfigParser()
>>> c["DEFAULT"] = {"ok": "value", "error": ""}
>>> buf = io.StringIO(); c.write(buf); buf.getvalue()
'[DEFAULT]\nok = value\nerror = \n\n'

This drops the space when the value is empty, giving error =.

Two cases it deliberately leaves alone:

• Delimiters that end in whitespace (delimiters=(' ',)). There the trailing whitespace is the separator, not padding, so stripping it would produce a bare key that cannot be read back. The guard checks self._delimiters[0] rather than the padded delimiter for exactly this reason.
• Values that genuinely end in whitespace ({"k": "v "} → k = v ). Stripping the whole line, as the issue suggests, would silently truncate them.

write() is the only caller of _write_section() and all three call sites pass the same delimiter, so one guard there covers every path; no sibling caller has the same bug. The behaviour dates to 96a60ae (2010), which introduced space_around_delimiters and concatenated the padded delimiter without an empty-value case - the trailing space looks like an artifact rather than a deliberate choice.

Output for non-empty values, for space_around_delimiters=False, and for valueless options under allow_no_value is byte-for-byte unchanged.

Fixes #157466

Testing

test_write_empty_value is added next to test_write, so it runs across the existing delimiter and parser variants (13 parameterisations). It pins both halves: the empty value loses the space, the whitespace-ending value keeps it. It fails on main in all 13 and passes with the change.

Also run: full test_configparser (396 tests), plus test_logging test_collections test_sysconfig test_venv test_importlib test_zipfile test_email (4,540 tests) - all pass. make patchcheck and the ruff-check configs CI uses for Lib/ and Lib/test/ are clean. IDLE is the other stdlib consumer that writes config files; its tests could not run here because _tkinter was not built, but idle_test.test_config only asserts file existence, not written text.

I did not touch the whitespace-delimiter round-trip failure visible in that area - that is #157456, and gh-157457 is already fixing it on the parse side.

Found with AI assistance; I reproduced and verified the behaviour and the fix on a locally built interpreter and understand them.

• Issue: configparser: whitespace not stripped when writing empty values #157466