Null vs Unknown in extractor helpers

Trigger: an extractor turns a types.Set/List/Map/Object into an SDK request body, and you see if x.IsNull() || x.IsUnknown() returning the same value.

Fix:

if x.IsUnknown() {
    return nil, nil               // omit key → server keeps existing value
}
if x.IsNull() {
    return []sdk.Foo{}, nil       // explicit empty → server clears on Replace
}

Why: the SDK's ToMap() uses IsNil() to decide whether to emit a JSON key. Collapsing both states either silently preserves removed values (both → nil) or silently destroys values still being resolved (both → empty slice).