[CLIENT-3411] Refactor code that initializes transaction-level as_policy_batch policy using a Python dictionary

[juliannguyen4]

POLICY_INIT(): remove policy init step because we are already copying the default values from the config level policy to the transaction level policy. (TODO: check this does not cause regressions for other policies) Set optional field variables to Python None by default. Setting them to NULL causes us to handle extra codepaths which is annoying to deal with