Proxy settings should not be set in /etc/freshclams.conf when node attributes are nil or empty

[haidangwa]

https://github.com/RoboticCheese/clamav-chef/blob/master/templates/default/freshclam.conf.erb#L110-L113

These lines should also check for #empty? and not enable proxy settings. We have a separate process that tries to determine proxy settings based on a node's location in the network. In particular, we have nodes that need outbound http proxy settings for other things, but not to get to clamav db mirrors. However, we can override node['clamav']['freshclam']['http_proxy_server'] and node['clamav']['freshclam']['http_proxy_port'] with empty String values in our roles and environment settings. Doing so results in a rendered /etc/freshclam.conf that has these settings enabled but with empty values. This causes a failure when the freshclam command is executed:

# freshclam
ERROR: Missing argument for option at line 106
ERROR: Can't open/parse the config file /etc/freshclam.conf