Fix for bug#58422: Incorrect result when OUTER JOIN'ing with an empty table
      
Fixed incorrect checks in join_read_const_table() for when to 
accept a non-existing, or empty const-row as a part of the
const'ified set of tables.
      
Intention of this test is to only accept NULL-rows if this table
is outer joined into the resultset. (In case of an inner-join
we can conclude at this point thatresultset will be empty,
and we want to return 'error' to signal this.)
      
Initially 'maybe_null' is set to the same value as 'outer_join'
in setup_table_map(), mysql_priv.h ~line 2424. Later
simplify_joins() will attemp to replace outer joins by inner 
join whenever possible.

This will cause 'outer_join' to be updated. However, 'maybe_null'
is *not* updated to reflect this rewrite as this field is used 
to currectly set the 'nullability' property for the columns 
in the resultset.
      
We should therefore change join_read_const_table() to check the
'outer_join' property instead of 'maybe_null', as this correctly
reflect the nullability of the partial *execution plan* (not *resultset*).