[SPARK-28003][PYTHON] Allow NaT values when creating Spark dataframe from pandas with Arrow

## What changes were proposed in this pull request?

This patch removes `fillna(0)` when creating ArrowBatch from a pandas Series.

With `fillna(0)`, the original code would turn a timestamp type into object type, which pyarrow will complain later:
```
>>> s = pd.Series([pd.NaT, pd.Timestamp('2015-01-01')])
>>> s.dtypes
dtype('<M8[ns]')
>>> s.fillna(0)
0                      0
1    2015-01-01 00:00:00
dtype: object
```

## How was this patch tested?

Added `test_timestamp_nat`

Closes apache#24844 from icexelloss/SPARK-28003-arrow-nat.

Authored-by: Li Jin <[email protected]>
Signed-off-by: Bryan Cutler <[email protected]>

Author: icexelloss

python/pyspark/serializers.py

@@ -296,8 +296,7 @@ def create_array(s, t):
             mask = s.isnull()
             # Ensure timestamp series are in expected form for Spark internal representation
             if t is not None and pa.types.is_timestamp(t):
-                s = _check_series_convert_timestamps_internal(s.fillna(0), self._timezone)
-
+                s = _check_series_convert_timestamps_internal(s, self._timezone)
             try:
                 array = pa.Array.from_pandas(s, mask=mask, type=t, safe=self._safecheck)
             except pa.ArrowException as e:

python/pyspark/sql/tests/test_arrow.py

@@ -383,6 +383,15 @@ def test_timestamp_dst(self):
         assert_frame_equal(pdf, df_from_python.toPandas())
         assert_frame_equal(pdf, df_from_pandas.toPandas())
 
+    # Regression test for SPARK-28003
+    def test_timestamp_nat(self):
+        dt = [pd.NaT, pd.Timestamp('2019-06-11'), None] * 100
+        pdf = pd.DataFrame({'time': dt})
+        df_no_arrow, df_arrow = self._createDataFrame_toggle(pdf)
+
+        assert_frame_equal(pdf, df_no_arrow.toPandas())
+        assert_frame_equal(pdf, df_arrow.toPandas())
+
     def test_toPandas_batch_order(self):
 
         def delay_first_part(partition_index, iterator):