I tested
SecFilterSelective HEADER_USER_AGENT "^$"
and
SecFilterSelective HEADER_USER_AGENT ""
not working.the test command is
#curl -A "" -I 127.0.0.1/test.html
however, if not empty, e.g.
SecFilterSelective HEADER_USER_AGENT "chome"
will work as expected -- will forbid Chrome browser to access.
however, you can use rewrite rule to archive the same goal, and much simpler.
RewriteCond %{HTTP_USER_AGENT} ^$
RewriteRule . - [F]
rewriterule is widely used and well tested, it's simpler yet powerful.