I want to branch from master and immediately create an empty Pull Request to discuss implementation details of a specific feature.

After selecting my branch and clicking "Pull Request", Github claims "Nothing to Compare" and doesn't offer any "Create Pull Request" button.

Github's own documentation on Flow states (click the right-arrow twice):

You can open a Pull Request at any point during the development process: when you have little or no code but want to share some screenshots or general ideas, when you're stuck and need help or advice, or when you're ready for someone to review your work.

But their own documentation on Pull Requests here states:

Pull requests can only be opened if there are differences between your branch and the upstream branch

I would like to utilise Github collaboration features with my team prior to committing changes to code. As we understand it, Pull Requests are for code changes and discussion of implementation details (ie: the "solution"), while Issues are higher level, for discussing features/bugs (ie: the "problem").

Am I misunderstanding these things?

Edit:

Apparently I can circumvent the process by changing the branch with an empty commit:

git commit --allow-empty -m "make pull request"

but I am trying to better understand why my (default) decision to create an empty PR was wrong in the first place, and why their documentation appears contradictory. Is there a strict "discuss code explicitly in PRs" policy or similar? Is this a contentious issue?

Update

Github have released Draft Pull Requests, but (alas) they can't be created if the 2 branches are identical.