[HUDI-1900] Always close the file handle for a flink mini-batch write (#2943)

Close the file handle eagerly to avoid corrupted files as much as
possible.

hudi-flink/src/test/java/org/apache/hudi/sink/TestWriteCopyOnWrite.java

@@ -500,9 +500,8 @@ public class TestWriteCopyOnWrite {
 
   Map<String, String> getMiniBatchExpected() {
     Map<String, String> expected = new HashMap<>();
-    expected.put("par1", "[id1,par1,id1,Danny,23,1,par1, "
-        + "id1,par1,id1,Danny,23,1,par1, "
-        + "id1,par1,id1,Danny,23,1,par1, "
+    // the last 3 lines are merged
+    expected.put("par1", "["
         + "id1,par1,id1,Danny,23,1,par1, "
         + "id1,par1,id1,Danny,23,1,par1]");
     return expected;

hudi-flink/src/test/java/org/apache/hudi/sink/utils/StreamWriteFunctionWrapper.java

@@ -163,6 +163,7 @@ public class StreamWriteFunctionWrapper<I> {
     functionInitializationContext.getOperatorStateStore().checkpointSuccess(checkpointId);
     coordinator.notifyCheckpointComplete(checkpointId);
     this.bucketAssignerFunction.notifyCheckpointComplete(checkpointId);
+    this.writeFunction.notifyCheckpointComplete(checkpointId);
     if (conf.getBoolean(FlinkOptions.COMPACTION_ASYNC_ENABLED)) {
       try {
         compactFunctionWrapper.compact(checkpointId);