fix(backend): Set comment to NULL instead of Denormalized

[Mar0xy]

What

The DROP migration listed setting the comment on the channelId column to the same as the UP migration which is invalid.

It should be NULL instead to clear out the comment

Initial concern about this was raised here https://activitypub.software/TransFem-org/Sharkey/-/merge_requests/537#note_3712

Why

(Look at the text above)

Checklist

• Read the contribution guide
• Test working in a local environment

[github-actions]

このPRによるapi.jsonの差分

差分はこちら

Get diff files from Workflow Page

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.24%. Comparing base (9849aab) to head (b2da70a).
Report is 297 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop   #13966       +/-   ##
============================================
- Coverage    77.84%   65.24%   -12.61%     
============================================
  Files          185     1009      +824     
  Lines        25600   115425    +89825     
  Branches       487     5496     +5009     
============================================
+ Hits         19929    75306    +55377     
- Misses        5664    38677    +33013     
- Partials         7     1442     +1435     

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[anatawa12]

https://github.com/typeorm/typeorm/blob/e7649d2746f907ff36b1efb600402dedd5f5a499/src/driver/postgres/PostgresQueryRunner.ts#L1138-L1151

It looks this is behavior of typeorm and there are meny similar down migrations so I think we don't have to fix this (if down migration works)

これはtypeormの実装起因っぽいですしすでにいろいろ似てるのあるし、(down migrationが動くなら)直さなくていい気がします

misskey/packages/backend/migration/1680931179228-account-move.js

Lines 16 to 21 in c51347d

	async down(queryRunner) {
	await queryRunner.query(`COMMENT ON COLUMN "user"."alsoKnownAs" IS 'URIs the user is known as too'`);
	await queryRunner.query(`ALTER TABLE "user" DROP COLUMN "alsoKnownAs"`);
	await queryRunner.query(`COMMENT ON COLUMN "user"."movedToUri" IS 'The URI of the new account of the User'`);
	await queryRunner.query(`ALTER TABLE "user" DROP COLUMN "movedToUri"`);
	}

misskey/packages/backend/migration/1607353487793-isExplorable.js

Lines 15 to 19 in c51347d

	async down(queryRunner) {
	await queryRunner.query(`DROP INDEX "IDX_d5a1b83c7cab66f167e6888188"`);
	await queryRunner.query(`COMMENT ON COLUMN "user"."isExplorable" IS 'Whether the User is explorable.'`);
	await queryRunner.query(`ALTER TABLE "user" DROP COLUMN "isExplorable"`);
	}

[Mar0xy]

I would have done t his just to keep consistency but oh well