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]

👍

• I tested this: Tested the internal PR and verified that the exact same changes were carried over.
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