stray empty node with unknown role

[pkra]

E.g., rendering \int \frac{a}{b} in https://zorkow.github.io/semantic-tree-visualiser/visualise.html gives a stray empty node without role at the end

This leads to empty speech when navigating which is confusing.

[pkra]

As per F2F with @zorkow this is not a bug. The reason I originally filed this was the empty speech that gets produced by this, i.e., a variant of #560.