Don't return null from ReplaceSelectionCommand::insertAsListItems() 

When trying to replace list by list, we insert list item instead of list itself, 
by calling insertAsListItems(). However, insertAsListItems() returns null when
it takes empty list.

This patch changes to ReplaceSelectionCommand::insertAsListItems() not to return null.

BUG=238170
TEST=LayoutTests/editing/execCommand/crash-replacing-list-by-list.html

Review URL: https://codereview.chromium.org/220233013

git-svn-id: svn://svn.chromium.org/blink/trunk@170821 bbb929c8-8fbe-4397-9dbb-9b2b20218538