Provide native way to cast Deps(Mut)<Custom> to Deps(Mut)<Empty>

[jawoznia]

    pub fn to_empty(self) -> QuerierWrapper<'a, Empty> {
        QuerierWrapper {
            querier: self.querier,
            custom_query_type: PhantomData,
        }
    }


impl<'a, C: CustomQuery> Deps<'a, C> {
    pub fn to_empty(self) -> Deps<'a, Empty> {
        Deps {
            storage: self.storage,
            api: self.api,
            querier: self.querier.to_empty(),
        }
    }
}