Preserve blank lines in option help text

[niukanen1]

Option help text already preserves explicit line breaks, but an empty line was
dropped because textwrap.wrap("") returns no output lines. This collapsed
separate help paragraphs in rendered pytest --help output.

Keep one empty formatted line for each empty input line. The integration
regression registers an option through pytest's Parser and verifies the
rendered help output. A formatter-level test also confirms that ordinary
nonempty wrapping is unchanged.

The contributor entry is included in AUTHORS, together with the numbered
changelog/15021.bugfix.rst entry.

Validation:

• pytest testing/test_parseopt.py — 35 passed on the current base.
• tox -e linting — passed on the current base.
• tox -e py313 — 4585 passed, 51 skipped, 15 xfailed, 5 xpassed on base a315b97. The subsequent rebase adds only upstream integration-plugin dependency pins; the code patch is unchanged.
• The changelog fragment passed its applicable pre-commit hooks.