do not error if auto-detect-proxy cmd has no output

Regression introduced in 8f858d560e3b7b475c623c4e242d1edce246025a.

Commands are probably better of always having output through as the
fall through to the generic proxy settings is likely not intended. As
documenting and implementing this more consistently is kind of a
regression through, it is split off into the next commit.

Closes: 827713

apt-pkg/contrib/proxy.cc

@@ -52,7 +52,7 @@ bool AutoDetectProxy(URI &URL)
       return _error->Error("ProxyAutoDetect command '%s' failed!", AutoDetectProxyCmd.c_str());
    char buf[512];
    if (PipeFd.ReadLine(buf, sizeof(buf)) == nullptr)
-      return _error->Error("Failed to read in AutoDetectProxy");
+      return true;
    PipeFd.Close();
    ExecWait(Child, "ProxyAutoDetect", true);
    auto const cleanedbuf = _strstrip(buf);

test/integration/test-apt-helper

@@ -74,7 +74,13 @@ E: Download Failed"
 test_apt_helper_detect_proxy() {
     # no proxy
     testsuccessequal "Using proxy '' for URL 'http://example.com/'" apthelper auto-detect-proxy http://example.com/
-
+    cat > apt-proxy-detect <<'EOF'
+#!/bin/sh -e
+exit 0
+EOF
+    chmod 755 apt-proxy-detect
+    echo "Acquire::http::Proxy-Auto-Detect \"$(pwd)/apt-proxy-detect\";" > rootdir/etc/apt/apt.conf.d/02proxy-detect
+    testsuccessequal "Using proxy '' for URL 'http://www.example.com/'" apthelper auto-detect-proxy http://www.example.com
 
     # http auto detect proxy script
     cat > apt-proxy-detect <<'EOF'