Simplify union node with empty input

[feilong-liu]

Description

Simplify a union node when it has at least one empty input but more than one non-empty input

Motivation and Context

Current optimization rule only simplify the union node when all input is empty or only one input is non-empty. This PR will simplify for the case where it has at least one empty input but more than one non-empty input.

Impact

Simplify for more query plans.

Test Plan

Unit test

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Release Notes

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== RELEASE NOTES ==

General Changes
* Fix the simplify plan with empty input rule to also work for union node with empty input but more than one non-empty input.