global: add null safety checks

Notesnook.API/Extensions/ClaimsPrincipalExtensions.cs

@@ -10,5 +10,8 @@ namespace System.Security.Claims
         private readonly static string[] SUBSCRIBED_CLAIMS = ["believer", "education", "essential", "pro", "legacy_pro"];
         public static bool IsUserSubscribed(this ClaimsPrincipal user)
          => user.Claims.Any((c) => c.Type == "notesnook:status" && SUBSCRIBED_CLAIMS.Contains(c.Value));
+
+        public static string GetUserId(this ClaimsPrincipal user)
+        => user.FindFirstValue("sub") ?? throw new Exception("User ID not found in claims.");
     }
 }