[python] Fix incorrect variable name for nullable fields with regex contraints

[changhc]

Fixes #15622. This is a tiny fix, so I didn't update the sample spec files to cover this but tested locally. If you consider it's worth updating the spec, I can do that.

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@spacether @krjakbrjak I would appreciate your review as member of the technical committee for python. Thanks!

[wing328]

@changhc thanks for the fix. I'll update the samples if needed.

[wing328]

FYI. Filed #15625 to add a test to catch the issue moving forward.