[IMP] models._auto_init: avoid writing `False` default for boolean fields

Boolean fields always default to False in 8.0,
even when they do not have explicit default values.
This causes extra queries in the form:
  UPDATE <table> SET <bool_field> = false
      WHERE <bool_field> IS NULL;

Those are not necessary as the ORM automatically
folds NULL booleans to False, and can be very
expensive on tables with several million rows,
as the whole table may sometimes need to be
rewritten (can take dozens of minutes)