Relax migration schema a bit

Co-authored-by: Fedor Indutny <79877362+indutny-signal@users.noreply.github.com>

ts/sql/migrations/1680-cleanup-empty-strings.std.ts

@@ -8,6 +8,7 @@ import { safeParseUnknown } from '../../util/schemas.std.js';
 
 const toNullable = z
   .string()
+  .or(z.null())
   .optional()
   .transform(value => value || undefined);
 

ts/test-node/sql/migration_1680_test.node.ts

@@ -88,7 +88,7 @@ describe('SQL/updateToSchemaVersion1680', () => {
           systemGivenName: '',
           systemFamilyName: '',
           systemNickname: '',
-          nicknameGivenName: '',
+          nicknameGivenName: null,
           nicknameFamilyName: '',
           username: '',