[1.4.4] Breaking Null tiles drops Null Tile Place-O-Matics

[stormytuna]

Due to some changes to 1.4.4 tModLoader, tiles will automatically drop an item that has its createTile field set to their type. This causes any Null tiles you break to produce 1 Null Tile Place-O-Matic, quickly filling up your inventory and cluttering the floor with items

[stormytuna]

Adding to this, Null Walls also drop Null Wall items

[shnakamura]

I fixed this in a PR, refer to #17