The predicate page_is_root(), which was added in MariaDB Server 10.2.2,
is based on a wrong assumption.

Under some circumstances, InnoDB can transform B-trees into a degenerate
state where a non-leaf page has no sibling pages. Because of this,
we cannot assume that a page that has no siblings is the root page.
This bug will be tracked as MDEV-19022.

Because of the bug that may affect many InnoDB data files, we must remove
and replace the wrong predicate. Using the wrong predicate can cause
corruption. A leaf page is not allowed to be empty except if it is the
root page, and the entire table is empty.