Made empty buckets be able to get pulled out of the coal generator

Closes #132

src/main/java/de/ellpeck/actuallyadditions/mod/tile/TileEntityCoalGenerator.java

@@ -105,7 +105,7 @@ public class TileEntityCoalGenerator extends TileEntityInventoryBase implements
 
     @Override
     public boolean canExtractItem(int slot, ItemStack stack, EnumFacing side){
-        return false;
+        return TileEntityFurnace.getItemBurnTime(this.slots[0]) <= 0;
     }
 
     @Override