The default value is not null, but an empty string. Here is the fix:
diff --git a/java/org/l2jmobius/gameserver/network/clientpackets/RequestChangePetName.java b/java/org/l2jmobius/gameserver/network/clientpackets/RequestChangePetName.java
index 650d1a6..1eaf439 100644
--- a/java/org/l2jmobius/gameserver/network/clientpackets/RequestChangePetName.java
+++ b/java/org/l2jmobius/gameserver/network/clientpackets/RequestChangePetName.java
@@ -53,7 +53,7 @@
return;
}
- if (pet.getName() != null)
+ if ((pet.getName() != null) && !pet.getName().equals(""))
{
player.sendPacket(SystemMessageId.YOU_CANNOT_SET_THE_NAME_OF_THE_PET);
return;