Handle null outputFileExtension in GetOutputFilePath

MediaBrowser.Api/Playback/BaseStreamingService.cs

@@ -131,6 +131,8 @@ namespace MediaBrowser.Api.Playback
         /// </summary>
         private string GetOutputFilePath(StreamState state, EncodingOptions encodingOptions, string outputFileExtension)
         {
+            if (outputFileExtension == null) outputFileExtension = "";
+            
             var data = $"{state.MediaPath}-{state.UserAgent}-{state.Request.DeviceId}-{state.Request.PlaySessionId}";
 
             var filename = data.GetMD5().ToString("N", CultureInfo.InvariantCulture);