[main.lua] fix empty rootPath cause try "/log" folder

When run follow command with get error message, 
```
$ bin/Linux/lua-language-server  -E main.lua
bin/Linux/lua-language-server: filesystem error: cannot create directories: Permission denied [/log]
```
Which is caused by empty `rootPath' and the folder path for log with be `/log' in follow line,
https://github.com/sumneko/lua-language-server/blob/1458139721646236406825a03a21152d98753515/main.lua#L40

This change try to assign a default value `"."' for `rootPath' will fix the error.