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 {};
      |             ^
```

0 files changed, 0 insertions, 0 deletions