Add fallback routines for empty secret cases

[juliusknorr]

Make sure to keep authentication working when an instance has been setup without a secret after adding the secret manually afterwards.

• Password auth to webdav still works
• App password auth to webdav still works
• Browser session keeps being active
• PublicKeyTokens get rotated if the fallback is hit so their private key is reencrypted with the secret now

Provides a possible migration path for https://github.com/nextcloud/server/pull/31492

ToDo

• [ ] Add migration step to update the config if possible and set a secret before the upgrade
  • [ ] secret
  • [ ] paswordsalt

[juliusknorr]

Test results when having an instance without a secret and adding one afterwards:

• Users get logged out of their sessions in the browser
• Reauthentication with existing password works -> password salt change is confirmed to not be an issue
• App passwords still fail

[juliusknorr]

Retested with follow up commits:

• [x] Password auth to webdav still works
• [x] App password auth to webdav still works
• [x] Browser session keeps being active
• [x] PublicKeyTokens get rotated if the fallback is hit so their private key is reencrypted with the secret now

[CarlSchwan]

Should we merge it without the migration path and just add the warning from https://github.com/nextcloud/server/pull/31492 instead?

It's better than the current state

[CarlSchwan]

/backport to stable25

[CarlSchwan]

Manual backport https://github.com/nextcloud/server/pull/35605