fix: prevent null start date in course details

[viadanna]

Description

This PR ensures a course cannot have an empty release date.

Ref: BB-9629

Testing instructions

1. Open Studio, navigate to a course, and set its release date to an empty string.
2. Ensure it isn't changed.

Deadline

None

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[kaustavb12]

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[feanil]

@viadanna I'm concerned about how generic this is. If other blocks have a "start" field, this behavior will also effect them right? Is that intended?

I think having this kind of logic that is specific to the course_details live in the course_details api makes more sense: https://github.com/openedx/edx-platform/blob/master/cms/djangoapps/contentstore/rest_api/v1/views/course_details.py#L121

But perhaps I'm missing something.

[viadanna]

@feanil If this is too generic, we can close the PR. The original intent was to fix an issue here, which was fixed in Teak by catching the AttributeError.