This looks different, but niche:
the tag is on the slice, and it wants to skip elements inside the slice if they are null.
Both the omitempty tag and the omitnil proposal only apply if the entire slice is nil/null.
I believe anything about omitempty is a red-herring. The OP is asking about a change to Unmarshal behavior, while omitempty and related issues are about Marshal behavior.
@mikolajsemeniuk, what is the representation of the Variant type? In order for the behavior you're asking for to make sense, there must be a representation for Variant that naturally corresponds with a JSON null.