Import errors in vulnerabilities.importers.gitlab.GitLabAPIImporter

```
parse_yaml_file: affected_range is not parsable: '(,)' type:'nuget' error: InvalidVersion("'None' is not a valid ")
Traceback (most recent call last):
File "/app/vulnerabilities/importers/gitlab.py", line 220, in parse_gitlab_advisory
affected_version_range = vrc.from_native(affected_range)
File "/usr/local/lib/python3.9/site-packages/univers/version_range.py", line 616, in from_native
VersionConstraint(comparator="=", version=cls.version_class(str(lower_bound)))
File "", line 7, in __init__
self.__attrs_post_init__()
File "/usr/local/lib/python3.9/site-packages/univers/versions.py", line 87, in __attrs_post_init__
raise InvalidVersion(f"{self.string!r} is not a valid {self.__class__!r}")
univers.versions.InvalidVersion: 'None' is not a valid

parse_yaml_file: affected_range is not parsable: '(,)' type:'nuget' error: InvalidVersion("'None' is not a valid ")
Traceback (most recent call last):
File "/app/vulnerabilities/importers/gitlab.py", line 220, in parse_gitlab_advisory
affected_version_range = vrc.from_native(affected_range)
File "/usr/local/lib/python3.9/site-packages/univers/version_range.py", line 616, in from_native
VersionConstraint(comparator="=", version=cls.version_class(str(lower_bound)))
File "", line 7, in __init__
self.__attrs_post_init__()
File "/usr/local/lib/python3.9/site-packages/univers/versions.py", line 87, in __attrs_post_init__
raise InvalidVersion(f"{self.string!r} is not a valid {self.__class__!r}")
univers.versions.InvalidVersion: 'None' is not a valid

Unknown package type: conan
```