NullPointerException on null unsafe Comparable implementations

There is a problem when comparing objects containing null unsafe Comparable imlementing classes.

TestCase:

```
class SomeObject {
    BigDecimal value;
    SomeObject(BigDecimal value) {
        this.value = value; 
    }
}

// produces NullPointerException
ObjectDifferFactory.getInstance(
    new SomeObject(BigDecimal.ONE), 
    new SomeObject(null));
```
