refactor: improve readability for AttemptSelection

it was pointed out by a few reviewers that the code block at the end
of attempt selection was difficult to follow and lacked comments.

refactor to get rid of triple nested if statement and improve
readibility.

src/wallet/spend.cpp

@@ -481,19 +481,20 @@ std::optional<SelectionResult> AttemptSelection(const CWallet& wallet, const CAm
             results.push_back(*result);
         }
     }
-
-    // If we can't fund the transaction from any individual OutputType, run coin selection
-    // over all available coins, else pick the best solution from the results
-    if (results.size() == 0) {
-        if (allow_mixed_output_types) {
-            if (auto result{ChooseSelectionResult(wallet, nTargetValue, eligibility_filter, available_coins.All(), coin_selection_params)}) {
-                return result;
-            }
+    // If we have at least one solution for funding the transaction without mixing, choose the minimum one according to waste metric
+    // and return the result
+    if (results.size() > 0) return *std::min_element(results.begin(), results.end());
+
+    // If we can't fund the transaction from any individual OutputType, run coin selection one last time
+    // over all available coins, which would allow mixing
+    if (allow_mixed_output_types) {
+        if (auto result{ChooseSelectionResult(wallet, nTargetValue, eligibility_filter, available_coins.All(), coin_selection_params)}) {
+            return result;
         }
-        return std::optional<SelectionResult>();
-    };
-    std::optional<SelectionResult> result{*std::min_element(results.begin(), results.end())};
-    return result;
+    }
+    // Either mixing is not allowed and we couldn't find a solution from any single OutputType, or mixing was allowed and we still couldn't
+    // find a solution using all available coins
+    return std::nullopt;
 };
 
 std::optional<SelectionResult> ChooseSelectionResult(const CWallet& wallet, const CAmount& nTargetValue, const CoinEligibilityFilter& eligibility_filter, const std::vector<COutput>& available_coins, const CoinSelectionParams& coin_selection_params)