fix: blank not null for StoredObject doc_name/doc_rev

[jennifer-richards]

Allows matching a doc via StoredObject.objects.filter(doc_name=doc.name, doc_rev=doc.rev), which doesn't work if those fields are None. Will want to confirm that the migration is not slow in staging.

Todo before promoting from draft PR:

• Confirm that StoredObject does not need explicit defaults for those fields
• Time migration in dev

If / when this is merged, we can roll back #10401.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@                Coverage Diff                @@
##             feat/storedobj   #10400   +/-   ##
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  Files                     ?      325           
  Lines                     ?    43644           
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  Hits                      ?    38566           
  Misses                    ?     5078           
  Partials                  ?        0           

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[jennifer-richards]

Migration in dev takes about 7 seconds total (with about 4.7 seconds of that in migration system overhead)

dev ➜ /workspace $ time ietf/manage.py migrate --plan
Planned operations:
doc.0031_storedobject_blank_not_null
    Alter field doc_name on storedobject
    Alter field doc_rev on storedobject
ietf/manage.py migrate --plan  3.33s user 1.06s system 94% cpu 4.653 total
dev ➜ /workspace $ time ietf/manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, blobdb, community, contenttypes, dbtemplate, django_celery_beat, django_celery_results, doc, group, iesg, ipr, liaisons, mailinglists, mailtrigger, meeting, message, name, nomcom, oidc_provider, person, redirects, review, sessions, sites, stats, status, submit, tastypie, utils
Running migrations:
  Applying doc.0031_storedobject_blank_not_null... OK
ietf/manage.py migrate  3.57s user 0.68s system 60% cpu 6.992 total