Empty villager trades are valid

Fixes #3171

core/src/main/java/org/geysermc/geyser/translator/protocol/java/inventory/JavaMerchantOffersTranslator.java

@@ -104,7 +104,9 @@ public class JavaMerchantOffersTranslator extends PacketTranslator<ClientboundMe
             recipe.put("sell", getItemTag(session, trade.getOutput()));
 
             // The buy count before demand and special price adjustments
-            recipe.putInt("buyCountA", Math.max(trade.getFirstInput().getAmount(), 0));
+            // The first input CAN be null as of Java 1.19.0/Bedrock 1.19.10
+            // Replicable item: https://gist.github.com/Camotoy/3f3f23d1f80981d1b4472bdb23bba698 from https://github.com/GeyserMC/Geyser/issues/3171
+            recipe.putInt("buyCountA", trade.getFirstInput() != null ? Math.max(trade.getFirstInput().getAmount(), 0) : 0);
             recipe.putInt("buyCountB", trade.getSecondInput() != null ? Math.max(trade.getSecondInput().getAmount(), 0) : 0);
 
             recipe.putInt("demand", trade.getDemand()); // Seems to have no effect