fix Gemma adapter without system role

Author: Nexesenex

kcpp_adapters/Gemma-2_without_system_role.json

@@ -0,0 +1,8 @@
+{
+"system_start":"",
+"system_end":"",
+"user_start":"<start_of_turn>user",
+"user_end":"<end_of_turn>\n",
+"assistant_start":"<start_of_turn>model",
+"assistant_end":"<end_of_turn>\n"
+}