Add README and build scripts

Program.cs

@@ -16,6 +16,10 @@ class Program
     static string PASS = "pass";
     static readonly string UPSTREAM_USER = Environment.GetEnvironmentVariable("UPSTREAM_USER");
     static readonly string UPSTREAM_PASS = Environment.GetEnvironmentVariable("UPSTREAM_PASS");
+    static readonly bool UPSTREAM_ALLOW_EMPTY_PASSWORD = string.Equals(
+        Environment.GetEnvironmentVariable("UPSTREAM_ALLOW_EMPTY_PASSWORD"),
+        "1",
+        StringComparison.OrdinalIgnoreCase);
     static readonly string UPSTREAM_PROXY_AUTH_HEADER = BuildUpstreamProxyAuthHeader();
     static long _connectionSeq;
 
@@ -363,6 +367,12 @@ class Program
         if (string.IsNullOrWhiteSpace(UPSTREAM_USER))
             return string.Empty;
 
+        if (string.IsNullOrEmpty(UPSTREAM_PASS) && !UPSTREAM_ALLOW_EMPTY_PASSWORD)
+        {
+            Log.Info("UPSTREAM_PASS is empty; upstream Basic auth header is disabled");
+            return string.Empty;
+        }
+
         string pass = UPSTREAM_PASS ?? string.Empty;
         string encoded = Convert.ToBase64String(Encoding.ASCII.GetBytes($"{UPSTREAM_USER}:{pass}"));
         return $"Proxy-Authorization: Basic {encoded}\r\n";

run.bat

@@ -12,10 +12,12 @@ if not exist "%EXE%" (
 set "UPSTREAM_PROXY=proxy.syktsu.ru:3128"
 set "UPSTREAM_USER=%USERNAME%"
 set "UPSTREAM_PASS="
+set "UPSTREAM_ALLOW_EMPTY_PASSWORD=0"
 
 echo [run] UPSTREAM_PROXY=%UPSTREAM_PROXY%
 echo [run] UPSTREAM_USER=%UPSTREAM_USER%
 echo [run] UPSTREAM_PASS is empty
+echo [run] UPSTREAM_ALLOW_EMPTY_PASSWORD=%UPSTREAM_ALLOW_EMPTY_PASSWORD%
 echo [run] Starting proxy...
 
 "%EXE%"