Fix bug when selection end is less than start

src/SelectionModel.test.ts

@@ -106,10 +106,6 @@ describe('SelectionManager', () => {
       model.selectionEnd = [1, 2];
       assert.equal(model.finalSelectionEnd, null);
     });
-    it('should return null if there is no selection end or selection start length', () => {
-      model.selectionStart = [1, 2];
-      assert.equal(model.finalSelectionEnd, null);
-    });
     it('should return selection start + length if there is no selection end', () => {
       model.selectionStart = [2, 2];
       model.selectionStartLength = 2;