Mongo’s 2.x driver allowed the credentials list to be null, however
the 3.x driver requires an empty list instead. The behaviour of the 2.x
driver is the same whether the client is created with a null credential
list or an empty credential list.

This commit aligns with the requirements of the 3.x driver by ensuring
that we never pass in a null credential list when creating the client.

Closes gh-4076