Fix IStructureElementChain returning an invalid Iterable (#33)

src/main/java/com/gtnewhorizon/structurelib/StructureLib.java

@@ -42,7 +42,7 @@ import cpw.mods.fml.relauncher.SideOnly;
 @Mod(
         modid = StructureLibAPI.MOD_ID,
         name = "StructureLib",
-        version = "GRADLETOKEN_VERSION",
+        version = Tags.VERSION,
         acceptableRemoteVersions = "*",
         guiFactory = "com.gtnewhorizon.structurelib.GuiFactory")
 public class StructureLib {

src/main/java/com/gtnewhorizon/structurelib/structure/IStructureElementChain.java

@@ -62,7 +62,8 @@ public interface IStructureElementChain<T> extends IStructureElement<T> {
             if (e == null) continue;
             if (predicate == null) predicate = e.getPredicate();
             else predicate = predicate.or(e.getPredicate());
-            is.add(e.getStacks());
+            Iterable<ItemStack> stacks = e.getStacks();
+            if (stacks != null) is.add(stacks);
         }
         if (predicate == null) return null;
         return new BlocksToPlace(predicate, Iterables.concat(is));