support removing attributes from animated elements (try 2)

[LoganDark]

Why

inert does not accept inert='', inert='false', inert='undefined' or etc. The attribute must be entirely removed in order to enable the element again. disabled works the same way.

What

If a value resolves to undefined (not null or false or otherwise) its attribute will now be entirely removed.

Checklist

• [x] Ready to be merged