AIP-83 when triggering run via new UI, default null, but give a date picker

[dstandish]

Body

In new UI, when triggering a run, it will be default None but you can select a date with date picker

reference: https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=337676722#Option2clarificationdocWIP-Resolutionofquestions

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