Don't require user when getting current session

Jellyfin.Api/Helpers/RequestHelpers.cs

@@ -120,7 +120,12 @@ public static class RequestHelpers
     internal static async Task<SessionInfo> GetSession(ISessionManager sessionManager, IUserManager userManager, HttpContext httpContext, Guid? userId = null)
     {
         userId ??= httpContext.User.GetUserId();
-        var user = userManager.GetUserById(userId.Value);
+        User? user = null;
+        if (!userId.IsNullOrEmpty())
+        {
+            user = userManager.GetUserById(userId.Value);
+        }
+
         var session = await sessionManager.LogSessionActivity(
             httpContext.User.GetClient(),
             httpContext.User.GetVersion(),