fix(cli): don't fail on workflows without inputs/outputs and print approriate warnings

[Panaetius]

closes #3654

[coveralls]

Pull Request Test Coverage Report for Build 7786040388

• -3 of 8 (62.5%) changed or added relevant lines in 2 files are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 85.148%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
renku/domain_model/workflow/plan.py	3	4	75.0%
renku/core/workflow/model/concrete_execution_graph.py	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
renku/core/dataset/providers/dataverse.py	1	64.76%
renku/ui/cli/init.py	1	96.95%
renku/ui/cli/service.py	1	78.31%

Totals
Change from base Build 7785635789:	-0.02%
Covered Lines:	26653
Relevant Lines:	31302

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💛 - Coveralls

[mohammad-alisafaee]

Thank you!