fix: validation API for certificates

[arslanashraf7]

Description

The validation API /api/courses/v1/validation/ fails with a 500 error (See the error logs below) when called with all=True and the FEATURES["CERTIFICATES_HTML_VIEW"]=False. This is a logical error in code that tried to check the len(None).

The pull request changes the default value of certificates=None to certificates=[] to standardize the method output, hence stopping the 500 error on len(certificates)>0.

Useful information to include:

These changes were made in #37047 when we migrated the views to class-based.

• This change impacts Course Author, Operator roles
• Which edX user roles will this change impact? Common user roles are "Learner", "Course Author",
  "Developer", and "Operator".

Supporting information

Internal Ticket: https://github.com/mitodl/hq/issues/9027

The validation API throws a 500 error under certain conditions mentioned above.
This is the error log:

TypeError: object of type 'NoneType' has no len()
(5 additional frame(s) were not displayed)
...
  File "cms/djangoapps/contentstore/api/views/utils.py", line 136, in _wrapper_view
    return view(self, request, course_key, *args, **kwargs)
  File "cms/djangoapps/contentstore/api/views/course_validation.py", line 101, in get
    certificates=self._certificates_validation(course)
  File "cms/djangoapps/contentstore/api/views/course_validation.py", line 223, in _certificates_validation
    has_certificate=certificates_enabled and len(certificates) > 0,

Testing instructions

• Set up a course in the studio with one or more non-audit course modes
• Set FEATURES["CERTIFICATES_HTML_VIEW"]=False in your environment
• If not already, enable the Authoring MFE and open the course
• You should not see the error (See attached screenshot)

Deadline

"None" if there's no rush, or provide a specific date or event (and reason) if there is one.

Other information

Error Screenshot

[openedx-webhooks]

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[pdpinch]

Can we have a test for this change?

[arslanashraf7]

Can we have a test for this change?

I couldn't find an existing test that tests this specific method, and since this was a logical error, I skipped it as well. I'll try to add one for this change.

[arslanashraf7]

@pdpinch I've updated an existing test with multiple conditions that should be able to cover this type of error if it happens in the future. This is ready for review again.

[marslanabdulrauf]

LGTM 👍

[pdpinch]

Does this need to be back-ported to Ulmo, or was the bug it fixes introduced after Ulmo was cut?

[arslanashraf7]

Does this need to be back-ported to Ulmo, or was the bug it fixes introduced after Ulmo was cut?

It was introduced in 54164a9 ~3mo ago. So, it was definitely there before Ulmo was cut. I guess this will need to be backported to Ulmo.

[pdpinch]

👍