omitempty for tags that marshal into zero val but should be nil

[gregbarasch]

💸 TL;DR

By default, if we marshal a map or slice from nil into yaml, then back into the map/slice, the value will not be the same!
Without omitempty, we marshal nil into the emptyval, then emptyval as is back into the struct. It is not a known guarantee for users that nil == empty.

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