Fix  MAGN-5323 List.UniqueItems will not work on lists with null values

This pull request fix defect [MAGN-5323 List.UniqueItems will not work on lists with null values](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5323)

``List.UniqueItems`` internally uses ``DistinctComparer`` to do comparison, but it is ambiguous to call overloaded ``DistinceComparer.Eq`` for two nulls. We can't directly compare two nulls neither (``null == null`` will throw ambiguity error, see [this discussion] (http://stackoverflow.com/questions/4730648/c-sharp-nullable-equality-operations-why-does-null-null-resolve-as-false), so here comparing them to null separately. 

Regression test case  ``ListTest.TestUniqueItemsForNullList`` added.

@sharadkjaiswal  for you.