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/test/fuzz/coinscache_sim.cpp

@@ -139,7 +139,7 @@ struct CacheLevel
  *
  * The initial state consists of the empty UTXO set.
  */
-class CoinsViewBottom final : public CCoinsView
+class CoinsViewBottom final : public CoinsViewEmpty
 {
     std::map<COutPoint, Coin> m_data;
 
@@ -153,11 +153,6 @@ public:
         return std::nullopt;
     }
 
-    uint256 GetBestBlock() const final { return {}; }
-    std::vector<uint256> GetHeadBlocks() const final { return {}; }
-    std::unique_ptr<CCoinsViewCursor> Cursor() const final { return {}; }
-    size_t EstimateSize() const final { return m_data.size(); }
-
     void BatchWrite(CoinsViewCacheCursor& cursor, const uint256&) final
     {
         for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)) {