fix: fix ValueError when updating a data quality monitoring schedule

[luke-gerschwitz]

Issue #, if available: #5001

Description of changes: Fix ValueError raised when updating a data quality monitoring schedule with a cron_schedule_expression of 'NOW'.

Testing done:

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[luke-gerschwitz]

Hi @zhaoqizqwang could you please review when you get the chance? Thanks 🙂

[benieric]

Can we add some unit tests

[benieric]

To fix the black format errors you can run - tox -e black-format

[sage-maker]

Integ test failure logs: https://github.com/aws/sagemaker-python-sdk/actions/runs/13127360276/job/36689546917?pr=5002