How to match a possible null parameter in Mockito

From the javadocs of any()

Since Mockito 2.1.0, only allow non-null String. As this
is a nullable reference, the suggested API to match
null wrapper would be isNull(). We felt this
change would make tests harness much safer that it was with Mockito
1.x.

So, the way to match nullable string arguments is explicit declaration:

nullable(String.class)