fix: when trades for zero are requested don't throw

I thought no one needed to request zero prices, and so I threw in that
case.  #4114 reveals a
situation where it will happen in ordinary circumstances.

Change the AMM code to respond with an empty amount when an empty
amount is offered, and vice versa.

packages/run-protocol/src/vpool-xyk-amm/constantProduct/swap.js

@@ -189,10 +189,10 @@ export const swap = (
   );
   assertGreaterThanZero(poolAllocation.Central, 'poolAllocation.Central');
   assertGreaterThanZero(poolAllocation.Secondary, 'poolAllocation.Secondary');
-  assert(
-    isGreaterThanZero(amountGiven) || isGreaterThanZero(amountWanted),
-    X`amountGiven or amountWanted must be greater than 0: ${amountWanted} ${amountGiven}`,
-  );
+
+  if (!isGreaterThanZero(amountGiven) && !isGreaterThanZero(amountWanted)) {
+    return noTransaction;
+  }
 
   if (!isWantedAvailable(poolAllocation, amountWanted)) {
     return noTransaction;

packages/run-protocol/test/amm/constantProduct/test-checkInvariants.js

@@ -308,13 +308,21 @@ test('getInputPrice zero input', t => {
     inputReserve: 320n,
     outputReserve: 50n,
     inputValue: 0n,
+    outputValue: 0n,
+  };
+  testGetInputPriceNoTrade(t, input, true);
+  testGetInputPriceNoTrade(t, input, false);
+});
+
+test('getOutputPrice zero output', t => {
+  const input = {
+    inputReserve: 320n,
+    outputReserve: 50n,
+    inputValue: 0n,
+    outputValue: 0n,
   };
-  const messageA =
-    'amountGiven or amountWanted must be greater than 0: {"brand":"[Alleged: BLD brand]","value":"[0n]"} {"brand":"[Alleged: RUN brand]","value":"[0n]"}';
-  testGetInputPriceThrows(t, input, messageA, true);
-  const messageB =
-    'amountGiven or amountWanted must be greater than 0: {"brand":"[Alleged: RUN brand]","value":"[0n]"} {"brand":"[Alleged: BLD brand]","value":"[0n]"}';
-  testGetInputPriceThrows(t, input, messageB, false);
+  testGetOutputPriceNoTrade(t, input, true);
+  testGetOutputPriceNoTrade(t, input, false);
 });
 
 test('getInputPrice big product', t => {

packages/run-protocol/test/amm/constantProduct/test-compareBondingCurves.js

@@ -199,9 +199,20 @@ test('getInputPrice zero input', t => {
     outputReserve: 50n,
     inputValue: 0n,
   };
-  const message =
-    'amountGiven or amountWanted must be greater than 0: {"brand":"[Alleged: BLD brand]","value":"[0n]"} {"brand":"[Alleged: RUN brand]","value":"[0n]"}';
-  getInputPriceThrows(t, input, message);
+
+  const expectedOutput = 0n;
+  testInputGetPrice(t, input, expectedOutput);
+});
+
+test('getOutputPrice zero output', t => {
+  const input = {
+    inputReserve: 320n,
+    outputReserve: 50n,
+    outputValue: 0n,
+  };
+
+  const expectedInput = 0n;
+  testGetOutputPrice(t, input, expectedInput);
 });
 
 test('getInputPrice big product', t => {