[BUG] ExtraChest Steal and Fill don't try to merge item stacks

[IceTank]

Describe the bug
When you have a container where every slot is filled with an item, then the steal and fill behavior on ExtraChest does not attempt to merge existing stacks.

To Reproduce

1. Place a shulker or chest
2. Fill every slot with a dummy item except one slot that is filled with a single piece of dirt
3. Put a stack or a partial stack of dirt in your inventory
4. Press the fill button on the chest GUI
5. Nothing happens

Expected behavior
Extra chest attempts to put any items from the inventory into the container that can fit.