MDL-35453 avoid subquering outer-joined columns

This prevents ORA-01799: a column may not be outer-joined to a subquery,
happening when an outer joined column is used for subquering.

Ideally the "m_quiz_attempts quiza" should be an INNER join, but as far as
conditions are multiple and some of them requre to show (NOT xxx) attempts
the OUTER join must be preserved.

That automatically implies that not "quiza" column can be used to match
a subquery. The modification simply avoids that restriction with an equivalent
and related subquery using EXISTs.