fix: send proxy-authorization even with empty password

[barjin]

Reqwest should try authenticating with the proxy on URLs with only username or only password. E.g. curl works like this, see example below:

# ---------------------vvvv----- only username, no password
$ curl -vvvv -x http://user@127.0.0.1:44199 http://httpbin.org/get
*   Trying 127.0.0.1:44199...
* Connected to (nil) (127.0.0.1) port 44199 (#0)
#-----------------------------------vvvv---- curl understands the credentials correctly
* Proxy auth using Basic with user 'user'
> GET http://httpbin.org/get HTTP/1.1
> Host: httpbin.org
#----------------------vvvvvvvvvvvvvv base64-encoded "user"
> Proxy-Authorization: Basic dXNlcjo=
> User-Agent: curl/7.81.0
> Accept: */*
> Proxy-Connection: Keep-Alive

Prerequisite for apify/impit#324

[seanmonstar]

Thanks!