Enable user without password

[dklimpel]

Replaces: matrix-org/synapse#16739

Closes: matrix-org/synapse#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

[CLAassistant]

All committers have signed the CLA.

[erikjohnston]

@dklimpel Sorry that I missed getting this merged before we forked 🤦

[erikjohnston]

Err, I have no idea why GH is closing this PR?!

[wrjlewis]

Have raised a ticket with GH to fix / understand what is happening here..

[dklimpel]

Should I create a new PR?