Make sure liftListItem doesn't crash when multiple items can't be merged
Make sure liftListItem doesn't crash when multiple items can't be merged

FIX: Fix a crash in `liftListItem` that happens when list items that can't be merged
are lifted together.

Issue https://github.com/ProseMirror/prosemirror/issues/1207

ProseMirror/prosemirror-schema-list