`access_right` might not be required

[ax3l]

In AMReX-Codes/amrex#4878, the validator says:

Schema validation errors were encountered.
  /github/workspace/.zenodo.json::$: 'access_right' is a required property

I think it assumes "access_right": "open" if not present, because releases are accepted: https://doi.org/10.5281/zenodo.2555438

[ax3l]

Which makes me wonder: should we throw warnings for keys like access_right and license if missing? They are not required, but certainly best practice to add.

I think GH Actions support warnings these days.