fix(service): fix cache not getting refreshed

[Panaetius]

the walrus library does not store None, but turns it into empty string, which would always fail the check.

[coveralls]

Pull Request Test Coverage Report for Build 6946727366

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• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 1014 unchanged lines in 84 files lost coverage.
• Overall coverage decreased (-3.3%) to 82.73%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
renku/ui/service/gateways/repository_cache.py	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
renku/core/dataset/providers/dataverse.py	1	88.86%
renku/core/dataset/providers/renku.py	1	86.69%
renku/core/image.py	1	97.14%
renku/core/init.py	1	92.86%
renku/core/migration/migrate.py	1	82.14%
renku/core/template/usecase.py	1	89.71%
renku/domain_model/provenance/agent.py	1	81.72%
renku/domain_model/template.py	1	90.43%
renku/ui/cli/init.py	1	96.9%
renku/ui/cli/service.py	1	28.57%

Totals
Change from base Build 6929940137:	-3.3%
Covered Lines:	25508
Relevant Lines:	30833

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💛 - Coveralls