Solve the compatibility problem of SeparatorStyle.CHATML type messes field

[icowan]

Why are these changes needed?

Solve the compatibility problem of SeparatorStyle.CHATML type messes field

/usr/local/lib/python3.10/dist-packages/fastchat/conversation.py", line 197, in get_prompt ERROR | stderr | ret += role + ":" + message + seps[i % 2] + "\n" ERROR | stderr | TypeError: can only concatenate str (not "NoneType") to str

Checks

• [x] I've run format.sh to lint the changes in this PR.
• [x] I've included any doc changes needed.
• [x] I've made sure the relevant tests are passing (if applicable).