Comet sink operator should not have children operators #25

Describe the bug

This fixes incorrect native operator when transforming Spark query plan to Comet query plan. We now add children operators into native operator anyway in operator2Proto. But for Comet sink operators e.g. shuffle, they don't have children operators but work as a input source. This doesn't cause issue because we don't actually plan these children for sink operators in native planner, but the native operator (proto message) looks incorrect and confusing.

Steps to reproduce

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Expected behavior

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Additional context

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