v2.8.0 submittable WorkflowTemplates are now required to have default values for all input parameters?

[samath117]

Checklist:

• I've included the version.
• I've included reproduction steps.
• I've included the workflow YAML.
• I've included the logs.

What happened:

After upgrading to Argo v2.8.0, I ran argo template create on submittable WorkflowTemplates (i.e. those with an entrypoint) that had previously worked fine in Argo v2.7.6. The ones with input parameters without default values failed, with a variant of the following error message:
Failed to create workflow template: templates.T inputs.parameters.X was not supplied
where T is the entrypoint template and X is the first parameter which was not supplied with a default input parameter.

Both removing the entrypoint line and providing a dummy default input variable to the template solved this issue. The former prevents the template from being submitted on the commandline with argo submit --from, though. The latter only has downsides in terms of maintainability / readability and has different requirements than workflows, so it adds another counterexample to the "any workflow can be turned into a WorkflowTemplate" mantra. Of course, this is also a breaking change; my templates that worked fine in v2.7.6 needed to be modified before I could use them in v2.8.0.

What you expected to happen:

Not providing a default value to a submittable WorkflowTemplate should not raise any concerns, let alone be fatal to the workflow. Just like with workflows, those parameters will have to be passed at invocation time.

How to reproduce it (as minimally and precisely as possible):

Even Argo's first example of a WorkflowTemplate cannot be created in v2.8.0 due to this bug:

$ argo template create https://raw.githubusercontent.com/argoproj/argo/master/examples/workflow-template/templates.yaml
2020/05/26 18:40:56 Failed to create workflow template: templates.whalesay-template inputs.parameters.message was not supplied

This surprised me, because I expected there to be a test involving the published examples...

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[samath117]

Looks like this is duplicating #2857, the fix for which has unfortunately not been merged into v2.8.0. Hopefully we'll get it in v2.8.1!