fix(backend): Pipeline and PipelineVersion Description column value should be optional.

[lianghao208]

Description of your changes:
fix #7568
Pipeline and PipelineVersion Description column value should be optional, if the Description column type set to not null, it will occur an error when inserting a pipeline record with empty description.

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[google-oss-prow]

Hi @lianghao208. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[chensun]

/ok-to-test

[chensun]

/lgtm

[google-oss-prow]

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