fix(utils): reject empty project name in parse_wheel_filename/parse_sdist_filename

[r266-tech]

parse_wheel_filename and parse_sdist_filename silently accept a filename whose distribution name is empty and return ("", Version(...)) instead of raising the documented InvalidWheelFilename / InvalidSdistFilename:

>>> from packaging.utils import parse_wheel_filename, parse_sdist_filename
>>> parse_wheel_filename("-1.0-py3-none-any.whl")
('', <Version('1.0')>, (), frozenset({<py3-none-any @ ...>}))
>>> parse_sdist_filename("-1.0.tar.gz")
('', <Version('1.0')>)

An empty name is invalid per the binary/source distribution-format specs, and canonicalize_name(..., validate=True) already rejects it — so the parsers are inconsistent with their own :raises: contract, and callers using them to validate artifact filenames get a silently-empty name instead of a clear parse error.

Fix

• wheel: _wheel_name_regex used * (zero-or-more), so it matched the empty string and the existing "Invalid project name" branch never fired; change it to +.
• sdist: add an explicit empty-name guard after the rpartition, before canonicalize_name.

Both raise the parser-specific exception (InvalidWheelFilename / InvalidSdistFilename, both ValueError subclasses) so back-compat holds; valid filenames are unaffected. Regression cases added to the existing empty-component parametrize lists.

Sibling of the empty-tag-component rejection in #1234.