coins: add explicit CoinsViewEmpty noop backend

Introduce `CoinsViewEmpty` as an explicit no-op `CCoinsView` implementation, and define its singleton accessor out of line in `coins.cpp` to avoid `-Wunique-object-duplication` in shared-library builds.`
Use it at call sites that intentionally want a no-op backend instead of constructing anonymous placeholder views.

`CCoinsViewTest` and `CoinsViewBottom` now inherit defaults from `CoinsViewEmpty` (e.g. the unused `EstimateSize()`, which now returns 0).

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/bench/ccoins_caching.cpp

@@ -26,8 +26,7 @@ static void CCoinsCaching(benchmark::Bench& bench)
     ECC_Context ecc_context{};
 
     FillableSigningProvider keystore;
-    CCoinsView coinsDummy;
-    CCoinsViewCache coins(&coinsDummy);
+    CCoinsViewCache coins{&CoinsViewEmpty::Get()};
     std::vector<CMutableTransaction> dummyTransactions =
         SetupDummyInputs(keystore, coins, {11 * COIN, 50 * COIN, 21 * COIN, 22 * COIN});