SNOW-1668862: [Local testing] df.write.save_as_table fails for columns with null

[tvdboom]

Please answer these questions before submitting your issue. Thanks!

1. What version of Python are you using?

   Python 3.11.6 (tags/v3.11.6:8b6ee5b, Oct 2 2023, 14:57:12) [MSC v.1935 64 bit (AMD64)]

2. What operating system and processor architecture are you using?

   Windows-10-10.0.22631-SP0

3. What are the component versions in the environment (pip freeze)?

   pandas==2.2.2
   snowflake-snowpark-python==1.22.1

4. What did you do?

import pandas as pd
from snowflake.snowpark import Session
from snowflake.snowpark.functions import lit

mock_session = Session.builder.config("local_testing", True).create()
test_data = mock_session.create_dataframe(pd.DataFrame({"a": [1, 2]}))

df = test_data.with_column("b", lit(None))
df.write.save_as_table(table_name="a", mode="truncate")

5. What did you expect to see?

   I expected the dataframe to be stored. Got an error instead.

  File "C:\repos\hippolib\test.py", line 40, in <module>
    df.write.save_as_table(table_name="a", mode="truncate")
  File "C:\repos\hippolib\venv\Lib\site-packages\snowflake\snowpark\_internal\telemetry.py", line 195, in wrap
    result = func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^
  File "C:\repos\hippolib\venv\Lib\site-packages\snowflake\snowpark\dataframe_writer.py", line 276, in save_as_table
    result = session._conn.execute(
             ^^^^^^^^^^^^^^^^^^^^^^
  File "C:\repos\hippolib\venv\Lib\site-packages\snowflake\snowpark\mock\_connection.py", line 563, in execute
    res = execute_mock_plan(plan, plan.expr_to_alias)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\repos\hippolib\venv\Lib\site-packages\snowflake\snowpark\mock\_plan.py", line 1106, in execute_mock_plan
    return entity_registry.write_table(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\repos\hippolib\venv\Lib\site-packages\snowflake\snowpark\mock\_connection.py", line 133, in write_table
    raise SnowparkLocalTestingException(
snowflake.snowpark.mock.exceptions.SnowparkLocalTestingException: NULL result in a non-nullable column