Fix sending forms with floodgate for 1.19.20 (#3217)

* Fix sending forms with floodgate

* Comment about 1.19.20

* Swapped if-else

Co-authored-by: Tim203 <mctim203@gmail.com>

core/src/main/java/org/geysermc/geyser/translator/protocol/java/JavaCustomPayloadTranslator.java

@@ -72,12 +72,19 @@ public class JavaCustomPayloadTranslator extends PacketTranslator<ClientboundCus
             String dataString = new String(data, 3, data.length - 3, Charsets.UTF_8);
 
             Form form = Forms.fromJson(dataString, type, (ignored, response) -> {
+                byte[] finalData;
+                if (response == null) {
+                    // Response data can be null as of 1.19.20 (same behaviour as empty response data)
+                    // Only need to send the form id
+                    finalData = new byte[]{data[1], data[2]};
+                } else {
                     byte[] raw = response.getBytes(StandardCharsets.UTF_8);
-                byte[] finalData = new byte[raw.length + 2];
+                    finalData = new byte[raw.length + 2];
 
                     finalData[0] = data[1];
                     finalData[1] = data[2];
                     System.arraycopy(raw, 0, finalData, 2, raw.length);
+                }
 
                 session.sendDownstreamPacket(new ServerboundCustomPayloadPacket(channel, finalData));
             });