Update cli_wallet to support unencrypted memo

[abitmore]

Description

It is possible to use unencrypted memo:

bitshares-core/libraries/protocol/include/graphene/protocol/memo.hpp

Lines 36 to 37 in 173ff3d

	* If @ref from == @ref to and @ref from == 0 then no encryption is used, the memo is public.
	* If @ref from == @ref to and @ref from != 0 then invalid memo data

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

bitshares-core/libraries/wallet/wallet_sign.cpp

Lines 181 to 183 in 1f3e2e9

	FC_ASSERT( _keys.count(memo->to) > 0 || _keys.count(memo->from) > 0,
	"Memo is encrypted to a key ${to} or ${from} not in this wallet.",
	("to", memo->to)("from",memo->from) );

bitshares-core/libraries/wallet/operation_printer.cpp

Lines 86 to 88 in 23263b0

	FC_ASSERT( wallet._keys.count(memo->to) > 0 || wallet._keys.count(memo->from) > 0,
	"Memo is encrypted to a key ${to} or ${from} not in this wallet.",
	("to", memo->to)("from",memo->from) );

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)
• 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.

CORE TEAM TASK LIST

• Evaluate / Prioritize Bug Report
• Refine User Stories / Requirements
• Define Test Cases
• Design / Develop Solution
• Perform QA/Testing
• Update Documentation