Handle nested empty annotations

At the moment we create an alien meta item if an annotation
wraps contents of zero length, however we also need to handle
nested empty annotations, i.e. where the contents is other
empty annotations (i.e. alien meta items).

As alien meta items simply preserve DOM elements we don't
mind what type of alien meta items they are, so this also fixes
the case of an annotation wrapping another alien meta
e.g. a comment or <meta> tag.

Bug: 48605

Change-Id: I98d88b341efcd6384ee1fda5d6d7e1e1acb107aa
Merge "Handle nested empty annotations"