http_proxy: a blank CURLOPT_USERAGENT should not be used in CONNECT

Extended test 80 to verify this.

Reported-by: Stefan Eissing
Fixes #12680
Closes #12681

lib/http_proxy.c

@@ -131,8 +131,8 @@ CURLcode Curl_http_proxy_create_CONNECT(struct httpreq **preq,
       goto out;
   }
 
-  if(!Curl_checkProxyheaders(data, cf->conn, STRCONST("User-Agent"))
-     && data->set.str[STRING_USERAGENT]) {
+  if(!Curl_checkProxyheaders(data, cf->conn, STRCONST("User-Agent")) &&
+     data->set.str[STRING_USERAGENT] && *data->set.str[STRING_USERAGENT]) {
     result = Curl_dynhds_cadd(&req->headers, "User-Agent",
                               data->set.str[STRING_USERAGENT]);
     if(result)

tests/data/test80

@@ -53,7 +53,7 @@ http-proxy
 HTTP 1.0 CONNECT with proxytunnel and proxy+host Basic authentication
 </name>
 <command>
-http://test.%TESTNUMBER:%HTTPPORT/we/want/that/page/%TESTNUMBER -p --proxy1.0 %HOSTIP:%PROXYPORT --user iam:myself --proxy-user youare:yourself
+http://test.%TESTNUMBER:%HTTPPORT/we/want/that/page/%TESTNUMBER -p --proxy1.0 %HOSTIP:%PROXYPORT --user iam:myself --proxy-user youare:yourself -A ""
 </command>
 <features>
 proxy
@@ -67,15 +67,13 @@ proxy
 CONNECT test.%TESTNUMBER:%HTTPPORT HTTP/1.0
 Host: test.%TESTNUMBER:%HTTPPORT
 Proxy-Authorization: Basic eW91YXJlOnlvdXJzZWxm
-User-Agent: curl/%VERSION
 Proxy-Connection: Keep-Alive
 
 </proxy>
 <protocol>
 GET /we/want/that/page/%TESTNUMBER HTTP/1.1
 Host: test.%TESTNUMBER:%HTTPPORT
 Authorization: Basic aWFtOm15c2VsZg==
-User-Agent: curl/%VERSION
 Accept: */*
 
 </protocol>