[HUDI-2000] Release file writer for merge handle #close (#3068)

Co-authored-by: 喻兆靖 <yuzhaojing@bilibili.com>

hudi-client/hudi-client-common/src/main/java/org/apache/hudi/io/HoodieMergeHandle.java

@@ -350,6 +350,7 @@ public class HoodieMergeHandle<T extends HoodieRecordPayload, I, K, O> extends H
 
       if (fileWriter != null) {
         fileWriter.close();
+        fileWriter = null;
       }
 
       long fileSizeInBytes = FSUtils.getFileSize(fs, newFilePath);