Don't use proxy in BingRequest sample if not defined

Using empty proxy URI results in failing to open the connection when using
WinHTTP, so the current code is broken out of the box.

Author: vadz

Release/samples/BingRequest/bingrequest.cpp

@@ -29,7 +29,7 @@ web::http::client::http_client_config client_config_for_proxy()
     wchar_t* pValue;
     size_t len;
     auto err = _wdupenv_s(&pValue, &len, L"http_proxy");
-    if (!err) {
+    if (!err && pValue) {
         std::unique_ptr<wchar_t, void(*)(wchar_t*)> holder(pValue, [](wchar_t* p) { free(p); });
         uri proxy_uri(std::wstring(pValue, len));
 #else