CURLOPT_PROXY does not have to be non-empty-string

# Bug report

As of https://github.com/phpstan/phpstan-src/pull/1719 CURLOPT_PROXY reports an error if set to a simple string.

AFAIK proxy can be empty string or null as well, as per https://curl.se/libcurl/c/CURLOPT_PROXY.html default is null, and in Composer we always set it to empty string when there is no proxy and that definitely works.

cc @staabm 