Disallow Classifer.classifier from being optional

[miketheman]

It's unlikely, but possible, that a Classifier.classifer with a None value is created and stored in the database.

from warehouse.classifiers.models import Classifier
c = Classifier(classifier=None)
db.add(c)
db.commit()

No error is raised.

There's no database constraint for this either, and Postgres will allow any number of NULL records to exist despite the unique constraint, since every NULL is treated as unique. Much special.

I think there's enough safety in the runtime system to not freak out if there's not a value there, but it's something that the database and model should reject. However the existence of a NULL in production would block the bin/release step where we sync the classifiers, with this kind of error:

...
  File "/opt/warehouse/src/warehouse/cli/classifiers.py", line 59, in sync
    classifier.ordering = sorted_classifiers.index(classifier.classifier)
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: None is not in list

Since the sync command gets all of them, regardless of their values (which I believe to be correct for this use case).

This is probably a relatively small change with a migration - and in production the table is only 867 records long, and is rarely updated (other than during a deploy if needed), so is unlikely to take many locks.