Check attachment path for null before use

This one is particularly nasty because the `Path` is not supposed to be nullable. However, fixing it in the parser and type constructor is beyond the scope of the 10.9 release. For now, just check with `IsNullOrEmpty`.

Signed-off-by: gnattu <gnattuoc@me.com>

MediaBrowser.MediaEncoding/Attachments/AttachmentExtractor.cs

@@ -89,7 +89,8 @@ namespace MediaBrowser.MediaEncoding.Attachments
             string outputPath,
             CancellationToken cancellationToken)
         {
-            var shouldExtractOneByOne = mediaSource.MediaAttachments.Any(a => a.FileName.Contains('/', StringComparison.OrdinalIgnoreCase) || a.FileName.Contains('\\', StringComparison.OrdinalIgnoreCase));
+            var shouldExtractOneByOne = mediaSource.MediaAttachments.Any(a => !string.IsNullOrEmpty(a.FileName)
+                                                                              && (a.FileName.Contains('/', StringComparison.OrdinalIgnoreCase) || a.FileName.Contains('\\', StringComparison.OrdinalIgnoreCase)));
             if (shouldExtractOneByOne)
             {
                 var attachmentIndexes = mediaSource.MediaAttachments.Select(a => a.Index);