v2.0.x : Fix None return annotation in quantity_input

[Cadair]

This is a backport of #7336

[astropy-bot]

Hi there @Cadair 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog and making sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

I noticed the following issue with this pull request:

• Changelog entry not present (or pull request number missing) and neither the Affects-dev nor the no-changelog-entry-needed label are set

Would it be possible to fix this? Thanks!

If there are any issues with this message, please report them here.

[bsipocz]

@Cadair - Please try to rebase as the LTS branch is not up to date with the backports.

Also, this will need the changelog, but that's in from the other PR so I can add the number before the release (the astropy-bot status fail is thus expected and should be ignored).

Once again, in future please do work in master only and rather split up PRs to backportable and non backportable parts rather than open PRs directly to bugfix.

[bsipocz]

The same CI issues are already on the branch, so I'm merging this now. Thanks @Cadair.