Merge pull request #2849 from lyonzy/patch-1

Handle null outputFileExtension in GetOutputFilePath

(cherry picked from commit 5c669d7ad7145eb2b54ec88a8636a28ea4eefcd1)
Signed-off-by: Joshua M. Boniface <joshua@boniface.me>

MediaBrowser.Api/Playback/BaseStreamingService.cs

@@ -134,7 +134,7 @@ namespace MediaBrowser.Api.Playback
             var data = $"{state.MediaPath}-{state.UserAgent}-{state.Request.DeviceId}-{state.Request.PlaySessionId}";
 
             var filename = data.GetMD5().ToString("N", CultureInfo.InvariantCulture);
-            var ext = outputFileExtension.ToLowerInvariant();
+            var ext = outputFileExtension?.ToLowerInvariant();
             var folder = ServerConfigurationManager.GetTranscodePath();
 
             if (EnableOutputInSubFolder)