maybe_total_supply in pallet-assets (paritytech#10799)

* maybe_total_supply in pallet-assets

* Update frame/assets/src/functions.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

frame/assets/src/functions.rs

@@ -55,7 +55,12 @@ impl<T: Config<I>, I: 'static> Pallet<T, I> {
 
 	/// Get the total supply of an asset `id`.
 	pub fn total_supply(id: T::AssetId) -> T::Balance {
-		Asset::<T, I>::get(id).map(|x| x.supply).unwrap_or_else(Zero::zero)
+		Self::maybe_total_supply(id).unwrap_or_default()
+	}
+
+	/// Get the total supply of an asset `id` if the asset exists.
+	pub fn maybe_total_supply(id: T::AssetId) -> Option<T::Balance> {
+		Asset::<T, I>::get(id).map(|x| x.supply)
 	}
 
 	pub(super) fn new_account(