修复向量知识库报错

yudao-module-llm/yudao-module-llm-biz/src/main/java/cn/iocoder/yudao/module/llm/service/http/RagHttpService.java

@@ -127,6 +127,9 @@ public class RagHttpService {
         detector.dataEnd();
         String charset = detector.getDetectedCharset();
         detector.reset();
+        if(charset == null){
+            return StandardCharsets.UTF_8.toString();
+        }
         return charset;
     }