Don't use null check operator on potentially nil value in ConnectionC…

…lose message

This commit attempts to fix issue #104 by using a ?? operator to avoid
raising a _CastError if a ConnectionClose message does not include a
replyText field.

lib/src/client/impl/client_impl.dart

@@ -163,7 +163,7 @@ class _ClientImpl implements Client {
         ConnectionClose serverResponse =
             (serverMessage.message as ConnectionClose);
         throw ConnectionException(
-            serverResponse.replyText!,
+            serverResponse.replyText ?? "Server closed the connection",
             ErrorType.valueOf(serverResponse.replyCode),
             serverResponse.msgClassId,
             serverResponse.msgMethodId);