Verification of an empty varargs call fails when isNotNull() is used

The follwing tests fail unexpected with : "Argument(s) are different"

```
@Test
public void shouldMatchVarArgs_isNotNull()   {
    mock.varargs();
    verify(mock).varargs(isNotNull());
}

@Test
public void shouldMatchVarArgs_isNotNullArray()   {
    mock.varargs();

    verify(mock).varargs((String[])isNotNull());
}
```

Expected is that the tests pass cause an empty vararg array is not null! 
