fix(backend): Allow empty namespace in pipelines. Fixes #8840.

[gkcalat]

Description of your changes:

• Removed namespace validation when converting to v2 API pipeline message

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• 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]

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[gkcalat]

/test all

[chensun]

/lgtm
/approve

[google-oss-prow]

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