HTTP_USER_AGENT is a HTTP header send by the client (browser).
Many browsers gimmick Mozilla (IE, Mozilla, Firefox, etc.)
The client sends an (optional) User-agent header like: Mozilla/4.0
If this string is empty, the system has no way to detect which browser is used and actually bans the client (optional, see admin security system).
We need the user-agent detection because there are big differences between a real browser and IE. The identification is mostly needed to workaround the many present IE bugs.
If a firewall, router, proxy or other software prevents the client from sending this header (it's optional anyway) then some guru decided to hide it, or else it is some malicious bot to spam/scam your website.
In either case the visitor is pretty useless to us and has no need to visit/harvest the website.
We could prevent the notice in the Apache logs but, it is pretty usefull to backtrace the nature of the client IP.
Server specs (Server OS / Apache / MySQL / PHP / DragonflyCMS):
Fedora 25 / Apache 2.4.27 / MariaDB 10.1.26 / PHP 7.1.10 / Mercurial