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/rpc/rawtransaction.cpp

@@ -623,8 +623,7 @@ static RPCMethod combinerawtransaction()
     CMutableTransaction mergedTx(txVariants[0]);
 
     // Fetch previous transactions (inputs):
-    CCoinsView viewDummy;
-    CCoinsViewCache view(&viewDummy);
+    CCoinsViewCache view{&CoinsViewEmpty::Get()};
     {
         NodeContext& node = EnsureAnyNodeContext(request.context);
         const CTxMemPool& mempool = EnsureMemPool(node);
@@ -638,7 +637,7 @@ static RPCMethod combinerawtransaction()
             view.AccessCoin(txin.prevout); // Load entries from viewChain into view; can fail.
         }
 
-        view.SetBackend(viewDummy); // switch back to avoid locking mempool for too long
+        view.SetBackend(CoinsViewEmpty::Get()); // switch back to avoid locking mempool for too long
     }
 
     // Use CTransaction for the constant parts of the