Allow empty string as version for find_compatible_in_house

[schaap]

The empty string is used in several locations to indicate the newest version. Interpret it as such instead of rejecting it as invalid.

This resolves the problem as reported in issue #2429, allowing virtualenv to correctly fall back on retrieving the newest wheel for bundled package that needed to be downloaded when the output of pip did not contain the package's location. See the issue for more details.

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• ran the linter to address style issues (tox -e fix_lint)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation