SPIGOT-5425: Prevent empty/air loot (again?)

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

paper-server/nms-patches/LootTable.patch

@@ -0,0 +1,10 @@
+--- a/net/minecraft/server/LootTable.java
++++ b/net/minecraft/server/LootTable.java
+@@ -39,6 +39,7 @@
+     public static Consumer<ItemStack> a(Consumer<ItemStack> consumer) {
+         return (itemstack) -> {
+             if (itemstack.getCount() < itemstack.getMaxStackSize()) {
++                if (itemstack.isEmpty()) return; // CraftBukkit
+                 consumer.accept(itemstack);
+             } else {
+                 int i = itemstack.getCount();