SDK - Skip attributes with missing values during PipelineMeta serialization

[Ark-kun]

Before:

{
  "name": "foo",
  "description": "",
  "inputs": [{"name": "tag", "description": "", "type": "", "default": null}],
  "outnputs": []
}

after:

{
  "name": "foo",
  "inputs": [{"name": "tag"}]
}

---

This change is 

[gaoning777]

How about leaving the value as empty instead of skipping it?

[Ark-kun]

/test kubeflow-pipeline-e2e-test

[Ark-kun]

How about leaving the value as empty instead of skipping it?

Hmm. The intent for this PR was to skip the values that are not set or empty. I think that it's better to only produce the minimal output that has only the required information.

[ajchili]

/lgtm

[Ark-kun]

/approve

[k8s-ci-robot]

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