instant_alter_column_possible(): Do not support instantaneous removal
of NOT NULL if the table needs to be rebuilt for removing the hidden
FTS_DOC_ID column. This is not ideal and should ultimately be fixed
properly in MDEV-17459.
This basically is a duplicate of MDEV-18219, proving that the
assertion was not relaxed correctly.
dict_index_t::in_instant_init: A debug flag that will only be set in
btr_cur_instant_init_low() in order to suppress the assertion failure
in rec_init_offsets() for that code path only.
MDEV-11369 (instant ADD COLUMN) introduced a regression in the case the
leftmost leaf page of the clustered index is empty except for the
hidden metadata record.
btr_pcur_store_position(): If the only record in the leftmost leaf page
is the metadata record, store the position before the first user record
in the tree.
