SPIGOT-2977: Pass setAmount(0) through to wrapped ItemStack.

This class needs to be rewritten with more consistent semantics and alignment with Minecraft losing null, but that is probably a job for 1.12

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java

@@ -147,10 +147,10 @@ public final class CraftItemStack extends ItemStack {
         if (handle == null) {
             return;
         }
+
+        handle.setCount(amount);
         if (amount == 0) {
             handle = null;
-        } else {
-            handle.setCount(amount);
         }
     }