SMZ3: fix item links with link_replacement

[Silvris]

What is this fixing or adding?

Worlds are expected to be able to have get_filler_item_name called without any fill steps being performed. SMZ3 is relying on self.junkItemNames, which doesn't exist until create_items. This just adds a set of defaults for the list that still gets replaced during create_items.

How was this tested?

Generated a two-player SMZ3 with the following item link.

  item_links:
    # Share part of your item pool with other players.
    - name: All
      item_pool:
        - Everything
      replacement_item: null
      link_replacement: true

On current main, this will fail with an AttributeError.

If this makes graphical changes, please attach screenshots.