Make ContentLibrary.learning_package non-nullable

[kdmccormick]

Make a data migration for it.

Delete content libraries for which this field is currently null.

[himanshu-sharmav]

Hi @kdmccormick I can do that, assign that to me.

[ormsbee]

@himanshu-sharmav: Thank you for the kind offer, but I've got this one in flight already with #37499

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