Work around GCC -Wmaybe-uninitialized false positive

Summary:
This works around a GCC bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80635

Test Plan:
With an affected GCC version:
  ninja all check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8630

src/txmempool.cpp

@@ -1005,7 +1005,7 @@ std::optional<CTxMemPool::txiter> CTxMemPool::GetIter(const TxId &txid) const {
     if (it != mapTx.end()) {
         return it;
     }
-    return std::optional<txiter>{};
+    return std::optional<txiter>{std::nullopt};
 }
 
 CTxMemPool::setEntries