Add support for non-table nodes in tables (e.g. aliens)

Rewrite table iterator to return null when table cells
are aliens, and undefined when it reaches the end.

Fix table actions and selection adjustment to account
for the table matrix containing null entries.

Bug: 72717
Change-Id: I9109d814cc7854e306d02d846865e024c709416d
Merge "Add support for non-table nodes in tables (e.g. aliens)"