_schema represented as key-value storage for various
values common for Tarantool, like next id for space creation.
SQL requires format to be fully specified for columns being
access. Unfortunatelly, _schema is inserted into _space
before _space's format is set and since DD triggers are
disabled during upgrade, format for _schema tuple in _space
stays the same. So, set value nullable field in upgrade,
regenerate initial snap, update tests.
Also, as far as _schema's tuple in _space is not updated:
relax fieldno check in sql.c