Draft: all: don't add proxy setting during normalization

It doesn't seem useful to always add the proxy setting to each connection profile that supports IP configuration. This normalization requires additional code and especially, it creates a new empty section in every new keyfile:

 # nmcli connection add type dummy ifname dummy1
 # cat /etc/NetworkManager/system-connections/dummy-dummy1.nmconnection
 [connection]
 id=dummy-dummy1
 uuid=8dd2bb73-b338-428f-ad6d-082c37a58566
 type=dummy
 interface-name=dummy1

 [dummy]

 [ipv4]
 method=disabled

 [ipv6]
 addr-gen-mode=default
 method=disabled

 [proxy]

Since an empty (with method=none) proxy setting is the same as no proxy setting, avoid the normalization.