[Bleeding] Ice in the nether fades to air. Fixes BUKKIT-5373

Previously, whenever BlockFadeEvent was called for ice melting, it returned
the new BlockState with a type of Material.STATIONARY_WATER. However, in
the Nether, ice melting does not form water, but is simply replaced by air
instead. This changes the event to use the proper BlockState based on
whether the block is located in the Nether or not.