Fix incorrect position when orderable base is set - Ignore the nil positions

This bug appears when we have orderable_base set to some value other than default (i.e. 1).

Suppose we have a new column :position introduced in our collection. Now, initially the position value will be nil for all the existing records.  In order to update them, we'll need to call :save on each record.

In this scenario, we should not consider rows having position: nil while calculating bottom_orderable_position.