support Message() without content param

[better629]

Features

• support define Message("")

Feature Docs

Influence

Result

(metagpt_310) MacBook-Pro:MetaGPT xxxx$ pytest tests/metagpt/serialize_deserialize/test_schema.py 
========================================================================================================= test session starts ==========================================================================================================
platform darwin -- Python 3.10.13, pytest-7.2.2, pluggy-1.3.0
rootdir: /Users/xxxx/work/demo-code/MetaGPT
plugins: asyncio-0.23.2, anyio-3.7.1, mock-3.11.1
asyncio: mode=strict
collected 2 items                                                                                                                                                                                                                      

tests/metagpt/serialize_deserialize/test_schema.py ..                                                                                                                                                                            [100%]

========================================================================================================== 2 passed in 0.76s ===========================================================================================================
(metagpt_310) MacBook-Pro:MetaGPT xxxx$ pytest tests/metagpt/test_schema.py 
========================================================================================================= test session starts ==========================================================================================================
platform darwin -- Python 3.10.13, pytest-7.2.2, pluggy-1.3.0
rootdir: /Users/xxxx/work/demo-code/MetaGPT
plugins: asyncio-0.23.2, anyio-3.7.1, mock-3.11.1
asyncio: mode=strict
collected 4 items                                                                                                                                                                                                                      

tests/metagpt/test_schema.py ....  
==================================================================================================== 4 passed, 3 warnings in 0.59s =====================================================================================================

Other