Quoting SDAM: Other Topology Types:

A client SHOULD emit a warning when, in the process of updating its topology description, it removes the last server from the topology.

Emitting a warning could better assist users in catching the following cases that could lead to an empty topology and non-functional application:

• A typo in the replicaSet URI option
• Connecting to a standalone server with a replicaSet URI option present. SDAM: Allowed Configuration Combinations allows drivers to support a Single topology type even if the replica set name is not-null, but libmongoc elects not to do that.