MDEV-24740: Selectivity for equi-join predicates not involed in ref access is not taken into account for join cardinality estimation

First the function was checking if there is a keyuse only then we were
trying to use the function to get estimates for the equi-join predicated.
This function can calculate the estimates for selectivity for an non-indexed
equi-join condition also.

Second change is done when we try to get the avg frequency of a column,
then we need to make sure that statistics are available in the statistical
tables. This check was missing and so get_column_avg_frequency() was
returning zero for columns.

Updated few tests results.