tx: disallow zero amount outputs for liquid.

Commit 8bc314f removed the output dust
threshold, but did not consider outputs of 0 satoshis. Prevent this
case and return a consistent error (invalid amount) for both BTC and
Liquid if a 0-valued output is given.

Author: jgriffiths

src/transaction_utils.cpp

@@ -807,9 +807,13 @@ namespace green {
             return;
         }
 
-        if (!is_liquid && !addressee.value("is_greedy", false)) {
+        if (!addressee.value("is_greedy", false)) {
             const auto satoshi = j_amountref(addressee);
-            if (satoshi < session.get_dust_threshold(asset_id_hex)) {
+            if (!satoshi.value()) {
+                // TODO: Allow 0 OP_RETURN.
+                throw user_error(res::id_invalid_amount);
+            }
+            if (!is_liquid && satoshi < session.get_dust_threshold(asset_id_hex)) {
                 // Output is below the dust threshold. TODO: Allow 0 OP_RETURN.
                 throw user_error(res::id_amount_below_the_dust_threshold);
             }