fix: handle empty certificate_available_date in schedule & details PUT

[Anas12091101]

Description

When the Certificate display behavior is set to Immediately upon passing or End date of course and the Save button is clicked, the API returns a 400 error with the following response:

[
  "Datetime has wrong format. Use one of these formats instead: YYYY-MM-DDThh:mm[:ss[.uuuuuu]][+HH:MM|-HH:MM|Z]."
]

The third option, A date after the course end date, works correctly since it includes a date in the request payload.

This was introduced by PR #38213: "feat: add AuthZ permissions for schedule and course details"), which added certificate_available_date to the date_fields set in _classify_update. Unlike the other date fields (start_date, end_date, etc.), the MFE sends certificate_available_date: "" (empty string) when no date is applicable. The existing guard if payload_value is not None does not cover empty strings, so DateTimeField.to_internal_value("") was called, raising a DRF ValidationError that propagated as a 400 response.

This PR fixes this by normalizing empty string to None for date fields in _classify_update before calling to_internal_value, since an empty string semantically means "no date". Additionally, allow_null=True was added to the certificate_available_date field in CourseDetailsSerializer, making it consistent with end_date, enrollment_end, and enrollment_start, all of which already allowed null.

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Testing instructions

. Go to the Schedule & Details page of an instructor-paced course.
2. Select Immediately upon passing or End date of course under certificate display behavior.
3. Click Save.
4. Verify that the change saves without any error

Deadline

"None"

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