SPIGOT-5476: Fix trading empty items

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

paper-server/nms-patches/MerchantRecipe.patch

@@ -27,3 +27,11 @@
  
      public MerchantRecipe(NBTTagCompound nbttagcompound) {
          this.rewardExp = true;
+@@ -73,6 +87,7 @@
+ 
+     public ItemStack getBuyItem1() {
+         int i = this.buyingItem1.getCount();
++        if (i <= 0) return ItemStack.a; // CraftBukkit - SPIGOT-5476
+         ItemStack itemstack = this.buyingItem1.cloneItemStack();
+         int j = Math.max(0, MathHelper.d((float) (i * this.demand) * this.priceMultiplier));
+