[css-ruby] Overhanging and auto-hiding

The current spec says:

> The ruby annotations related to a ruby base must never overhang another
ruby base.

However, there are cases that a ruby base doesn't not actually have any
annotation with it. One of the cases would be auto-hidden annotation. Also,
if there is shortage of annotation, a ruby base could have no annotation to
pair with.

I propose that we should relax the restriction here, and allow ruby
annotation to overhang ruby base if the ruby base doesn't have effective
related annotation. Ruby base with no visible annotation should just be
treated as plain text.

- Xidorn