Invalid version range in `apache_httpd` and `gentoo` advisory

We should not create version range using empty constraint.

```python
'Cannot create AffectedPackage with invalid or unknown range: {'package': {'type': 'apache', 'namespace': '', 'name': 'httpd', 'version': '', 'qualifiers': '', 'subpath': ''}, 'affected_version_range': 'vers:apache/', 'fixed_version_range': None, 'introduced_by_commit_patches': [], 'fixed_by_commit_patches': []} with error:
Traceback (most recent call last):
File "/app/vulnerabilities/importer.py", line 564, in from_dict
VersionRange.from_string(affected_range) if affected_range else None
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.12/site-packages/univers/version_range.py", line 139, in from_string
raise ValueError(f"{vers!r} specifies no version range constraints.")
ValueError: 'vers:apache/' specifies no version range constraints.
```