FIX:getting data post creation in test_bulk_update_different_column_t…

[NandanaRaol]

This pull request includes a minor update to the test_bulk_update_different_column_types test in testapp/tests/test_expressions.py. The primary change involves modifying the handling of objects created via bulk_create to ensure they can be properly updated and tested for nullable fields.

Changes to test logic:

• Updated the bulk_create call to fetch the created objects again using a query, as bulk_create does not return the id of the created objects. Added comments explaining the need to reset field values to None for testing bulk_update with nullable fields.