refactor: return std::nullopt instead of {}

In #21415 we decided to return `std::optional` rather than `{}` for
uninitialized values. This PR repalces the two remaining usages of `{}`
with `std::nullopt`.

As a side-effect, this also quells the spurious GCC 10.2.x warning that
we've had reported quite a few times. i.e #21318, #21248, #20797.

```bash
txmempool.cpp: In member function ‘CTxMemPool::setEntries CTxMemPool::GetIterSet(const std::set<uint256>&) const’:
txmempool.cpp:898:13: warning: ‘<anonymous>’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  898 |     return {};
      |             ^
```

Author: fanquake

src/txmempool.cpp

@@ -895,7 +895,7 @@ std::optional<CTxMemPool::txiter> CTxMemPool::GetIter(const uint256& txid) const
 {
     auto it = mapTx.find(txid);
     if (it != mapTx.end()) return it;
-    return {};
+    return std::nullopt;
 }
 
 CTxMemPool::setEntries CTxMemPool::GetIterSet(const std::set<uint256>& hashes) const

src/validation.cpp

@@ -5186,7 +5186,7 @@ std::optional<uint256> ChainstateManager::SnapshotBlockhash() const {
         // If a snapshot chainstate exists, it will always be our active.
         return m_active_chainstate->m_from_snapshot_blockhash;
     }
-    return {};
+    return std::nullopt;
 }
 
 std::vector<CChainState*> ChainstateManager::GetAll()