MDEV-15060 Assertion in row_log_table_apply_op after instant ADD when the table is emptied during subsequent ALTER TABLE

During an online table rebuild, a table could be emptied and converted
from 'instant ADD' format to plain (pre-10.3) format. All online_log
records for rebuilding the table must be written and parsed in the
format of the table that existed at the start of the operation.

row_log_t::n_core_fields: A new field for recording index->n_core_fields
when online ALTER is initiated in row_log_allocate().

row_log_t::is_instant(): Determine if the log is in the instant format.
Only invoked by the row_log_table_ family of functions.

dict_index_t::get_n_nullable(): Remove is_instant() debug assertions.
Because a table can be converted to non-instant format during a
table-rebuilding ALTER TABLE, these assertions would be bogus when
executing row_log_table_apply().

rec_init_offsets_temp(): Add the parameter n_core for passing the
original index->n_core_fields.

rec_init_offsets_temp(): Add a 3-parameter variant.

rec_init_offsets_comp_ordinary(): Add the parameter n_core for
passing the index->n_core_fields.