[Core] Add specific exception for Stranger not found (#52)

Author: A-n-o-n-0

Lagrange.Core/Internal/Services/System/FetchStrangerService.cs

@@ -91,6 +91,12 @@ protected override ValueTask<FetchStrangerEventResp> Parse(ReadOnlyMemory<byte>
             property => property.Value
         );
 
+        // Check exists
+        if (!bytes.TryGetValue(20002, out byte[]? nicknameBytes))
+        {
+            throw new OperationException(-1, "Stranger not found");
+        }
+
         // Birthday
         byte[] birthday = bytes[20031];
         int year = BinaryPrimitives.ReadUInt16BigEndian(birthday.AsSpan(0, 2));
@@ -99,7 +105,7 @@ protected override ValueTask<FetchStrangerEventResp> Parse(ReadOnlyMemory<byte>
 
         return ValueTask.FromResult(new FetchStrangerEventResp(new BotStranger(
             response.Body.Uin,
-            Encoding.UTF8.GetString(bytes[20002]),
+            Encoding.UTF8.GetString(nicknameBytes),
             string.Empty, // Can't not get uid
             Encoding.UTF8.GetString(bytes[102]),
             Encoding.UTF8.GetString(bytes[103]),