Fix inconsistent $proxy_host handling in apt and apt::ppa.

- The default for $proxy_host is undef
- apt considers $proxy_set to be absent if $proxy_host is undef
- apt::ppa considers proxy_env to be empty if $proxy_host is false or ''

This results in apt::ppa to consider $proxy_host to be set when the default undef is used
breaking ppa resources because $proxy_env becomes:
  [http_proxy=http://:8080, https_proxy=http://:8080]

Fix this by making both apt and apt::ppa consider $proxy_host to be unset when it is
any of false, '', or undef.