[CBR-462] Fix fee estimation when transaction's amount matches accoun…

…t's balance

When calling

```
reduceChangeOutputs
  :: forall dom. CoinSelDom dom
  => Fee dom
  -> [Value dom]
  -> ([Value dom], Fee dom)
```

If the `[Value dom]` is a singleton array with a zero coin, we're going
to end up dividing by zero and silently obliterating fee as we do it.
This happens when making a transaction with no change, i.e., when the
transaction's amount matches exactly the source account's balance.

This commit provides a fix handling this special edge-case.

wallet-new/src/Cardano/Wallet/Kernel/CoinSelection/Generic.hs

@@ -590,16 +590,30 @@ nLargestFromListBy f n = \xs ->
     dropOne (Just [_])    = Nothing
     dropOne (Just (_:as)) = Just as
 
--- | Proportionally divide the fee over each output
+-- | Proportionally divide the fee over each output.
+--
+-- There's a special 'edge-case' when the given input is a singleton list
+-- with one 0 coin. This is artifically created during input selection when
+-- the transaction's amount matches exactly the source's balance.
+-- In such case, we can't really compute any ratio for fees and simply return
+-- the whole fee back with the given change value.
 divvyFee :: forall dom a. CoinSelDom dom
           => (a -> Value dom) -> Fee dom -> [a] -> [(Fee dom, a)]
-divvyFee _ _   [] = error "divvyFee: empty list"
-divvyFee f fee as = map (\a -> (feeForOut a, a)) as
+divvyFee _ _   []                     = error "divvyFee: empty list"
+divvyFee f fee [a] | f a == valueZero = [(fee, a)]
+divvyFee f fee as                     = map (\a -> (feeForOut a, a)) as
   where
     -- All outputs are selected from well-formed UTxO, so their sum cannot
     -- overflow
     totalOut :: Value dom
-    totalOut = unsafeValueSum (map f as)
+    totalOut =
+        let
+            total = unsafeValueSum (map f as)
+        in
+            if total == valueZero then
+                error "divyyFee: invalid set of coins, total is 0"
+            else
+                total
 
     -- The ratio will be between 0 and 1 so cannot overflow
     feeForOut :: a -> Fee dom