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innobase_add_instant_try(): If the leftmost leaf page does not contain
other records than the 'default row', only empty the table if there
are no successor pages.

When a table or partition which was not empty during a previous
instant ADD COLUMN became empty later, and now with this subsequent
instant ADD COLUMN we have the opportunity to convert the empty table
or partition to 'non-instant' format.

Similarly, if the table or partition is empty to begin with, that is,
it does not even contain a 'default row' record, we can use the
'non-instant' format.