comply with dotnet-5

Jellyfin.Api/Helpers/DynamicHlsHelper.cs

@@ -515,7 +515,7 @@ namespace Jellyfin.Api.Helpers
                 && state.VideoStream != null
                 && state.VideoStream.Level.HasValue)
             {
-                levelString = state.VideoStream.Level.ToString();
+                levelString = state.VideoStream.Level.ToString() ?? string.Empty;
             }
             else
             {
@@ -557,7 +557,7 @@ namespace Jellyfin.Api.Helpers
             }
             else if (!string.IsNullOrEmpty(codec))
             {
-                profileString = state.GetRequestedProfiles(codec).FirstOrDefault();
+                profileString = state.GetRequestedProfiles(codec).FirstOrDefault() ?? string.Empty;
                 if (string.Equals(state.ActualOutputVideoCodec, "h264", StringComparison.OrdinalIgnoreCase))
                 {
                     profileString = profileString ?? "high";

Jellyfin.Api/Helpers/HlsHelpers.cs

@@ -100,8 +100,9 @@ namespace Jellyfin.Api.Helpers
         /// <returns>The string text of #EXT-X-MAP.</returns>
         public static string GetFmp4InitFileName(string outputPath, StreamState state, bool isOsDepends)
         {
+            var directory = Path.GetDirectoryName(outputPath) ?? throw new ArgumentException($"Provided path ({outputPath}) is not valid.", nameof(outputPath));
             var outputFileNameWithoutExtension = Path.GetFileNameWithoutExtension(outputPath);
-            var outputPrefix = Path.Combine(Path.GetDirectoryName(outputPath), outputFileNameWithoutExtension);
+            var outputPrefix = Path.Combine(directory, outputFileNameWithoutExtension);
             var outputExtension = GetSegmentFileExtension(state.Request.SegmentContainer);
 
             // on Linux/Unix