Return error if asset amount is zero

This is primarily for the sake of symmetry
(`pool_join_with_exact_pool_amount` raises `MathApproximation` if the
asset amount is zero). Furthermore, calling this extrinsic with zero as
parameter is rather non-sensical, but, more importantly, this check
ensures that on-one can ever get free pool tokens.

zrml/swaps/src/lib.rs

@@ -1554,6 +1554,7 @@ mod pallet {
             asset_amount: BalanceOf<T>,
             min_pool_amount: BalanceOf<T>,
         ) -> Result<Weight, DispatchError> {
+            ensure!(asset_amount != Zero::zero(), Error::<T>::MathApproximation);
             let pool = Pallet::<T>::pool_by_id(pool_id)?;
             let pool_ref = &pool;
             let pool_account_id = Pallet::<T>::pool_account_id(pool_id);

zrml/swaps/src/tests.rs

@@ -571,6 +571,18 @@ fn pool_amount_must_not_be_zero() {
     });
 }
 
+#[test]
+fn asset_amount_must_not_be_zero() {
+    ExtBuilder::default().build().execute_with(|| {
+        create_initial_pool_with_funds_for_alice(ScoringRule::CPMM, true);
+
+        assert_noop!(
+            Swaps::pool_join_with_exact_asset_amount(alice_signed(), 0, ASSET_A, 0, 0),
+            crate::Error::<Runtime>::MathApproximation
+        );
+    });
+}
+
 #[test]
 fn pool_exit_decreases_correct_pool_parameters() {
     ExtBuilder::default().build().execute_with(|| {