Implementation is by Robbert <FP/iris-atomic!5 (comment 19496)>

get rid of the empty-unicode-space hacks

See merge request !59

This makes it easier to frame or introduce some modalities before introducing
universal quantifiers.

That way, we can use Contractive as part of structure definitions in
which we do not have a bundled OFE yet.

It can be derived, thanks to Ales for noticing!