GSSAPI Authentication is beginning the SASL conversation with an empty payload.

In 3.6 and before, MongoDB would return

{ "conversationId" : 1, "done" : false, "payload" : new BinData(0, ""), "ok" : 1.0 }

for an empty saslStart payload.

In 4.0, MongoDB passes the empty client payload per SASL spec to Kerberos which starts negotiation.

The C# driver should not pass an empty payload unless it wants to start SPNEGO.

If the C# driver does start calling saslStart with a non-empty payload, this is compatible with all versions of MongoDB.