Override htacces rule only for specific directory

To effectively disable that condition for that one URL-path, you will need to add another rule immediately before the rule in question. ie. before the # Abuse Agent Blocking comment.

For example:

# Allow an empty User-Agent for requests that start "/wp-content/uploads/xmls"
RewriteCond %{HTTP_USER_AGENT} ^$
RewriteRule ^wp-content/uploads/xmls - [S=1]

# Abuse Agent Blocking
RewriteCond %{HTTP_USER_AGENT} ^[Ww]eb[Bb]andit [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^$ [NC,OR]
RewriteCond %{HTTP_USER_AGENT} ^Acunetix [NC,OR]
: etc.

The first rule skips the following rule if the requested URL-path starts /wp-content/uploads/xmls and the User-Agent header is empty. So, it effectively bypasses the rule that would otherwise block it.

This does assume that the request maps to a physical file in that directory. ie. Not a request that would otherwise be rewritten to the WordPress front-controller (index.php).