change default value

apps/app/src/server/service/config-loader.ts

@@ -761,20 +761,20 @@ const ENV_VAR_NAME_TO_CONFIG_INFO: Record<string, EnvConfig> = {
     ns: 'crowi',
     key: 'openai:serviceType',
     type: ValueType.STRING,
-    default: null,
+    default: undefined,
   },
   OPENAI_API_KEY: {
     ns: 'crowi',
     key: 'openai:apiKey',
     type: ValueType.STRING,
-    default: null,
+    default: undefined,
     isSecret: true,
   },
   OPENAI_SEARCH_ASSISTANT_INSTRUCTIONS: {
     ns: 'crowi',
     key: 'openai:searchAssistantInstructions',
     type: ValueType.STRING,
-    default: null,
+    default: undefined,
   },
   /* eslint-disable max-len */
   OPENAI_CHAT_ASSISTANT_INSTRUCTIONS: {