Use string.IsNullOrEmpty instead of regular null check.

MediaBrowser.MediaEncoding/Subtitles/SubtitleEncoder.cs

@@ -537,7 +537,7 @@ namespace MediaBrowser.MediaEncoding.Subtitles
 
             foreach (var subtitleStream in subtitleStreams)
             {
-                if (subtitleStream.Path is null || !subtitleStream.Path.EndsWith(".mks", StringComparison.OrdinalIgnoreCase))
+                if (string.IsNullOrEmpty(subtitleStream.Path) || !subtitleStream.Path.EndsWith(".mks", StringComparison.OrdinalIgnoreCase))
                 {
                     continue;
                 }
@@ -608,7 +608,7 @@ namespace MediaBrowser.MediaEncoding.Subtitles
 
             foreach (var subtitleStream in subtitleStreams)
             {
-                if (subtitleStream.Path is not null && subtitleStream.Path.EndsWith(".mks", StringComparison.OrdinalIgnoreCase))
+                if (!string.IsNullOrEmpty(subtitleStream.Path) && subtitleStream.Path.EndsWith(".mks", StringComparison.OrdinalIgnoreCase))
                 {
                     _logger.LogDebug("Subtitle {Index} for file {InputPath} is part in an MKS file. Skipping", inputPath, subtitleStream.Index);
                     continue;