Fix MerchantOffer BuyB Only AssertionError (#6206)

patches/server/0728-Fix-MerchantOffer-BuyB-Only-AssertionError.patch

@@ -0,0 +1,18 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: thamid-gamer <60953955+thamid-gamer@users.noreply.github.com>
+Date: Fri, 16 Jul 2021 16:00:17 -0400
+Subject: [PATCH] Fix MerchantOffer BuyB-Only AssertionError
+
+
+diff --git a/src/main/java/net/minecraft/world/item/trading/MerchantOffer.java b/src/main/java/net/minecraft/world/item/trading/MerchantOffer.java
+index 70b703b920752e7301e4f19cdc07a1a4ceac5e0e..33660209885a9d56b127ca3926b6c7c60469127e 100644
+--- a/src/main/java/net/minecraft/world/item/trading/MerchantOffer.java
++++ b/src/main/java/net/minecraft/world/item/trading/MerchantOffer.java
+@@ -231,6 +231,7 @@ public class MerchantOffer {
+         if (!this.satisfiedBy(firstBuyStack, secondBuyStack)) {
+             return false;
+         } else {
++            if (!this.getCostA().isEmpty()) // Paper
+             firstBuyStack.shrink(this.getCostA().getCount());
+             if (!this.getCostB().isEmpty()) {
+                 secondBuyStack.shrink(this.getCostB().getCount());