Prefer null values over empty strings #28832

for everyone working on PRs for this issue, some hints :)

1. one PR for each column
2. do a migration to convert the database entries to null
3. check whether null and '' actually mean the same for that column
4. transform '' to null in the request schema to not break clients sending '' + add TODO comments to drop this in v4
5. add HistoryBuilder info on request fields indicating that clients must send null instead of '' in an upcoming major version
6. keep returning '' instead of null on the API surface to not break clients + add TODO comments to return null in v4
7. add HistoryBuilder info on response fields indicating that clients will send null instead of '' in an upcoming major version
8. update internal c…