(This is not a real fix for this bug, even though it makes it to no longer repeat)
- Semi-join subquery predicates, i.e. ... WHERE outer_expr IN (SELECT ...) may have null-rejecting properties,
  may allow to convert outer joins into inner.
- When convert_subq_to_sj() injected IN-equality into parent's WHERE/ON clause, it didn't call 
  $new_cond->top_level_item(), which would cause null-rejecting properties to be lost.
- Fixed, now the mentioned outer-to-inner conversion will really take place.