opt: avg size should be weighted by number of non-NULL rows

[mgartner]

See this comment: #86460 (review)

While we're here, though, we could fix an existing issue with average size calculation. The value in system.table_statistics.avgSize and returned by stat.AvgSize() is only for non-NULL values. We should probably weight it by the number of non-NULL rows, so that it is an average over all rows, instead of only being an average over non-NULL rows. I.e. it should be something like stats.AvgColSizes[cols.SingleColumn()] = stat.AvgSize() * (stat.RowCount() - stat.NullCount()) / stat.RowCount().

Jira issue: CRDB-18820