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/node/psbt.cpp

@@ -116,8 +116,7 @@ PSBTAnalysis AnalyzePSBT(PartiallySignedTransaction psbtx)
 
         // Estimate the size
         CMutableTransaction mtx(*psbtx.tx);
-        CCoinsView view_dummy;
-        CCoinsViewCache view(&view_dummy);
+        CCoinsViewCache view{&CoinsViewEmpty::Get()};
         bool success = true;
 
         for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {