Update cli_wallet to support unencrypted memo

**Description**

It is possible to use unencrypted memo:
https://github.com/bitshares/bitshares-core/blob/173ff3d27a4ca076a62bffdcd22d8f315b239a68/libraries/protocol/include/graphene/protocol/memo.hpp#L36-L37

However, `cli_wallet` does not support it (btw: code duplication).

https://github.com/bitshares/bitshares-core/blob/1f3e2e9b952f2978ef137bee60a9f6a7d9625571/libraries/wallet/wallet_sign.cpp#L181-L183

https://github.com/bitshares/bitshares-core/blob/23263b0e649ed657b79dd7168f046416d07cd9d0/libraries/wallet/operation_printer.cpp#L86-L88

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by this bug. Please tick at least one box.
- [ ] API (the application programming interface)
- [ ] Build (the build process or something prior to compiled code)
- [x] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [ ] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

**Additional Context (optional)**
[Add any other context about the problem here.
](https://github.com/bitshares/bitsharesjs/issues/8)

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Bug Report
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start by reading memo.hpp and compare the memo handling in libraries/wallet/wallet_sign.cpp with libraries/wallet/operation_printer.cpp. Trace how cli_wallet processes unencrypted memos, then verify that unencrypted memo operations are accepted and displayed correctly.

Written by the indexing model from the issue text.