SPIGOT-4940: Fletcher villager gifts sometimes crash server

nms-patches/LootTable.patch

@@ -0,0 +1,14 @@
+--- a/net/minecraft/server/LootTable.java
++++ b/net/minecraft/server/LootTable.java
+@@ -40,6 +40,11 @@
+ 
+     public static Consumer<ItemStack> a(Consumer<ItemStack> consumer) {
+         return (itemstack) -> {
++            // CraftBukkit start - SPIGOT-4940: no empty loot
++            if (itemstack.isEmpty()) {
++                return;
++            }
++            // CraftBukkit end
+             if (itemstack.getCount() < itemstack.getMaxStackSize()) {
+                 consumer.accept(itemstack);
+             } else {