Merge #21498: refactor: return std::nullopt instead of {}

5294f0d5a94cc7beaf692131fba0cad8beec9f13 refactor: return std::nullopt instead of {} (fanquake)

Pull request description:

  In #21415 [we decided](https://github.com/bitcoin/bitcoin/pull/21415#issuecomment-800236640) to return `std::optional` rather than `{}` for
  uninitialized values. This PR replaces 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 {};
        |             ^
  ```

ACKs for top commit:
  hebasto:
    ACK 5294f0d5a94cc7beaf692131fba0cad8beec9f13, I have reviewed the code and it looks OK, I agree it can be merged.

Tree-SHA512: 5b776be79ab26e5a3a5fc2b463b394ea5ce6797ed5558424873fa4ecee2898170eff76d6da9d69394d28f8f98974117fc63b922a3e19c52f5294c83073e79bb0

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