How to block User-Agent and blank / empty User-Agent Strings

As I know we can block bots with bots.d/blacklist-user-agents.conf files but does not understanding that what code need add in blacklist-user-agents.conf files. Please give me code for User-Agent "Yandex" and "Baidu", Also we have lot of block blank User-Agent strings is this good or need block? If so how we can block?

Please note, User-Agent- Majestic-12 and linkdexbot/2.0 still are not blocked.

该提问来源于开源项目：mitchellkrogza/nginx-ultimate-bad-bot-blocker