fix: change current_provider default value to nil

[CALUGC]

The PR is based on issue #87
The main cause of #87 is because current_provider in state.lua is initial as { chat = "", command = ""}, when chat_handler.lua ChatHandler:get_provider first time been called, current_prov will be nil so it will try to get prov by called State:get_provider, but it will return "" by self.file_state.current_provider.chat (or self.file_state.current_provider.command) because the file_state hasn't updated yet.
And since empty string is truthy, so chat_handler.lua:78 (if not prov) will be false, then finally lead to query a provider with empty string.

One of the solutions I came up with was to simply set current_provider to { chat = nil, command = nil } when init states.

[frankroeder]

Wow @CALUGC, thank you very much for further investigation and fixing it!