wallet: if only have one output type, don't perform "mixed" coin selection

there is nothing to mix.

src/wallet/spend.cpp

@@ -524,8 +524,9 @@ std::optional<SelectionResult> AttemptSelection(const CWallet& wallet, const CAm
     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
+    // over all available coins, which would allow mixing.
-    if (allow_mixed_output_types) {
+    // If TypesCount() <= 1, there is nothing to mix.
+    if (allow_mixed_output_types && available_coins.TypesCount() > 1) {
         if (auto result{ChooseSelectionResult(wallet, nTargetValue, eligibility_filter, available_coins.All(), coin_selection_params)}) {
             return result;
         }

src/wallet/spend.h

@@ -46,6 +46,8 @@ struct CoinsResult {
     /** The following methods are provided so that CoinsResult can mimic a vector,
      * i.e., methods can work with individual OutputType vectors or on the entire object */
     size_t Size() const;
+    /** Return how many different output types this struct stores */
+    size_t TypesCount() const { return coins.size(); }
     void Clear();
     void Erase(const std::unordered_set<COutPoint, SaltedOutpointHasher>& coins_to_remove);
     void Shuffle(FastRandomContext& rng_fast);