Don't allow empty password to be entered when using non-anon account.

Fixes getting wall of exception text when accidentally entering or pasting
in an empty string.

DepotDownloader/Program.cs

@@ -110,10 +110,13 @@ namespace DepotDownloader
             ContentDownloader.Config.MaxServers = Math.Max( ContentDownloader.Config.MaxServers, ContentDownloader.Config.MaxDownloads );
 
             if ( username != null && password == null && ( !ContentDownloader.Config.RememberPassword || !ConfigStore.TheConfig.LoginKeys.ContainsKey( username ) ) )
+            {
+                do
                 {
                     Console.Write("Enter account password for \"{0}\": ", username);
                     password = Util.ReadPassword();
                     Console.WriteLine();
+                } while ( String.Empty == password );
             }
             else if ( username == null )
             {