Refactor: how variant options in Interlinear correctly

made a method from duplicate code.
Also fix MorphItems.Sort() to handle null values.

LT-14486
difference in variants in interlinear between
7.2.7. and 8.0; can't use variant analysis?

Change-Id: I5b8a4f9c448e2515a09e83a94c0411a9427841ca