When overloaded methods present, the most specific method will be mapped for given method call. For this, out of all overloaded versions one version should be identified as more specific than all other.
The value 'null' can be assigned to both String and Object. But String type is close match than Object, since String extends Object.
Now, if you look at the method call (null, null) the first argument will match the second method (String, Object) over first, where as the second argument will match first method (String, Object) over second.