Fix arrow-json writer empty

The Writer output with the ArrayJson formatter was empty
when no record were in the output (no write or empty batch write).
It should output an empty JSON array `[]` instead.

arrow-json/src/writer/mod.rs

@@ -367,10 +367,15 @@ where
     /// all record batches have been produced. (e.g. producing the final `']'` if writing
     /// arrays.
     pub fn finish(&mut self) -> Result<(), ArrowError> {
-        if self.started && !self.finished {
+        if !self.started {
+            self.format.start_stream(&mut self.writer)?;
+            self.started = true;
+        }
+        if !self.finished {
             self.format.end_stream(&mut self.writer)?;
             self.finished = true;
         }
+
         Ok(())
     }
 
@@ -1147,12 +1152,45 @@ mod tests {
     }
 
     #[test]
-    fn json_writer_empty() {
+    fn json_line_writer_empty() {
+        let mut writer = LineDelimitedWriter::new(vec![] as Vec<u8>);
+        writer.finish().unwrap();
+        assert_eq!(str::from_utf8(&writer.into_inner()).unwrap(), "");
+    }
+
+    #[test]
+    fn json_array_writer_empty() {
         let mut writer = ArrayWriter::new(vec![] as Vec<u8>);
         writer.finish().unwrap();
+        assert_eq!(str::from_utf8(&writer.into_inner()).unwrap(), "[]");
+    }
+
+    #[test]
+    fn json_line_writer_empty_batch() {
+        let mut writer = LineDelimitedWriter::new(vec![] as Vec<u8>);
+
+        let array = Int32Array::from(Vec::<i32>::new());
+        let schema = Schema::new(vec![Field::new("c", DataType::Int32, true)]);
+        let batch = RecordBatch::try_new(Arc::new(schema), vec![Arc::new(array)]).unwrap();
+
+        writer.write(&batch).unwrap();
+        writer.finish().unwrap();
         assert_eq!(str::from_utf8(&writer.into_inner()).unwrap(), "");
     }
 
+    #[test]
+    fn json_array_writer_empty_batch() {
+        let mut writer = ArrayWriter::new(vec![] as Vec<u8>);
+
+        let array = Int32Array::from(Vec::<i32>::new());
+        let schema = Schema::new(vec![Field::new("c", DataType::Int32, true)]);
+        let batch = RecordBatch::try_new(Arc::new(schema), vec![Arc::new(array)]).unwrap();
+
+        writer.write(&batch).unwrap();
+        writer.finish().unwrap();
+        assert_eq!(str::from_utf8(&writer.into_inner()).unwrap(), "[]");
+    }
+
     #[test]
     fn json_struct_array_nulls() {
         let inner = ListArray::from_iter_primitive::<Int32Type, _, _>(vec![