Fix crash when mining BlockMultipart with Atomic Disassembler

Not sure if this is a Bug in Mekanism or in FMP, but I get the following crash when mining a multipart block with the Atomic Disassembler:
```
java.lang.NullPointerException: Ticking memory connection
	at net.minecraft.item.ItemStack.getItemDamage(ItemStack.java:266)
	at mekanism.api.ItemInfo.get(ItemInfo.java:19)
	at mekanism.common.OreDictCache.getOreDictName(OreDictCache.java:23)
	at mekanism.common.util.MekanismUtils.getOreDictName(MekanismUtils.java:486)
	at mekanism.common.item.ItemAtomicDisassembler.onBlockStartBreak(ItemAtomicDisassembler.java:104)
	at net.minecraft.server.management.ItemInWorldManager.tryHarvestBlock(ItemInWorldManager.java:289)
	at net.minecraft.server.management.ItemInWorldManager.uncheckedTryHarvestBlock(ItemInWorldManager.java:234)
	at net.minecraft.network.NetHandlerPlayServer.processPlayerDigging(NetHandlerPlayServer.java:529)
	at net.minecraft.network.play.client.C07PacketPlayerDigging.processPacket(C07PacketPlayerDigging.java:61)
	at net.minecraft.network.play.client.C07PacketPlayerDigging.processPacket(C07PacketPlayerDigging.java:94)
	at net.minecraft.network.NetworkManager.processReceivedPackets(NetworkManager.java:232)
	at net.minecraft.network.NetworkSystem.networkTick(NetworkSystem.java:182)
	at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:720)
	at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:608)
	at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:118)
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:482)
	at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:746)
```
I was able to trace it back to the ItemStack generated in ItemAtomicDisassembler#onBlockStartBreak(...)
```java
ItemStack stack = new ItemStack(block, 1, meta);
``` 
returning null for #getItem() when block is an instance of BlockMultipart, so checking for null in OreDictCache#getOreDictName() probably won't hurt ;-)