Allow reactivate a user without password

[matrixbot]

This issue has been migrated from #16739.

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Closes: #10397

An administrator should know whether he wants to set a password or not.
There are many uses cases where a blank password is required.

• Use of only some users with SSO.
• Use of bots with password, users with SSO

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

Signed-off-by: Dirk Klimpel 5740567+dklimpel@users.noreply.github.com

[dklimpel]

This issue was a PR.
And is fixed by

• Enable user without password #16770