Allow None for Decimal type conversion (specific to PyArrow 0.9.0)

Traceback (most recent call last):
  File "/.../spark/python/pyspark/sql/tests.py", line 4672, in test_vectorized_udf_null_decimal
    self.assertEquals(df.collect(), res.collect())
  File "/.../spark/python/pyspark/sql/dataframe.py", line 533, in collect
    sock_info = self._jdf.collectToPython()
  File "/.../spark/python/lib/py4j-0.10.7-src.zip/py4j/java_gateway.py", line 1257, in __call__
    answer, self.gateway_client, self.target_id, self.name)
  File "/.../spark/python/pyspark/sql/utils.py", line 63, in deco
    return f(*a, **kw)
  File "/.../spark/python/lib/py4j-0.10.7-src.zip/py4j/protocol.py", line 328, in get_return_value
    format(target_id, ".", name), value)
Py4JJavaError: An error occurred while calling o51.collectToPython.
: org.apache.spark.SparkException: Job aborted due to stage failure: Task 3 in stage 1.0 failed 1 times, most recent failure: Lost task 3.0 in stage 1.0 (TID 7, localhost, executor driver): org.apache.spark.api.python.PythonException: Traceback (most recent call last):
  File "/.../spark/python/pyspark/worker.py", line 320, in main
    process()
  File "/.../spark/python/pyspark/worker.py", line 315, in process
    serializer.dump_stream(func(split_index, iterator), outfile)
  File "/.../spark/python/pyspark/serializers.py", line 274, in dump_stream
    batch = _create_batch(series, self._timezone)
  File "/.../spark/python/pyspark/serializers.py", line 243, in _create_batch
    arrs = [create_array(s, t) for s, t in series]
  File "/.../spark/python/pyspark/serializers.py", line 241, in create_array
    return pa.Array.from_pandas(s, mask=mask, type=t)
  File "array.pxi", line 383, in pyarrow.lib.Array.from_pandas
  File "array.pxi", line 177, in pyarrow.lib.array
  File "error.pxi", line 77, in pyarrow.lib.check_status
  File "error.pxi", line 77, in pyarrow.lib.check_status
ArrowInvalid: Error converting from Python objects to Decimal: Got Python object of type NoneType but can only handle these types: decimal.Decimal