setup-proxy: fix empty proxy

the while loop loops until the value of proxyurl is none or a http*://
scheme. however, when proxyurl is set to 'none', it then gets unset here
(proxyurl= ;), but the following if block checks if it is 'none' and
fails, so we configure an empty http_proxy= in the proxy.sh profile
script. this is then read by some software, and it causes it to crash.
for instance, flatpak via glib reads http_proxy from the environment,
and an empty variable is illegal.

fixes aports#13899 (closed)