SignalR: Reconnecting event sends a null Exception object inside the handler.

### Describe the bug
Hello,
I'm using Microsoft.AspNetCore.SignalR.Client 3.1.
I found the following Reconnecting event behavior:
in some cases, the Exception argument of the handler for this event is null.

### To Reproduce
When I reproduced this error, I stopped the server in VS and after that the Reconnecting event handler on the Client had an Exception object = null.
I've seen this several times, but in most cases the Exception object is not null, so it's hard to write exact steps to reproduce.