tests: allow axfer of 0 amount to anyone

I believe this change would also support sending an axfer
of 0 amount to yourself (opt in) whilst also including a
close-out to yourself (close out) in a single axfer

Author: nullun

data/transactions/logic/evalAppTxn_test.go

@@ -468,6 +468,14 @@ func TestAppAxfer(t *testing.T) {
 			"assert failed") // app account not opted in
 
 		ledger.NewAccount(appAddr(888), 10000) // plenty for fees
+
+		// It should be possible to send 0 amount of an asset (existing
+		// or not) to any account but ourself. Regardless of being opted in
+		test("global CurrentApplicationAddress; txn Accounts 1; int 0" + axfer + "int 1")
+		holding, err := ledger.AssetHolding(appAddr(888), 77)
+		require.ErrorContains(t, err, "no asset 77 for account")
+		require.Equal(t, uint64(0), holding.Amount)
+
 		ledger.NewHolding(appAddr(888), 77, 3000, false)
 		test("global CurrentApplicationAddress; int 77; asset_holding_get AssetBalance; assert; int 3000; ==;")
 

data/transactions/logic/ledger_test.go

@@ -731,17 +731,20 @@ func (l *Ledger) axfer(from basics.Address, xfer transactions.AssetTransferTxnFi
 	}
 	fholding, ok := fbr.holdings[aid]
 	if !ok {
-		if from == to && amount == 0 {
-			// opt in
-			if params, exists := l.assets[aid]; exists {
-				fbr.holdings[aid] = basics.AssetHolding{
-					Frozen: params.DefaultFrozen,
+		if amount == 0 {
+			if from == to {
+				// opt in
+				if params, exists := l.assets[aid]; exists {
+					fbr.holdings[aid] = basics.AssetHolding{
+						Frozen: params.DefaultFrozen,
+					}
+				} else {
+					return fmt.Errorf("Asset (%d) does not exist", aid)
 				}
-				return nil
 			}
-			return fmt.Errorf("Asset (%d) does not exist", aid)
+		} else {
+			return fmt.Errorf("Sender (%s) not opted in to %d", from, aid)
 		}
-		return fmt.Errorf("Sender (%s) not opted in to %d", from, aid)
 	}
 	if fholding.Frozen {
 		return fmt.Errorf("Sender (%s) is frozen for %d", from, aid)