This template parameter serves to detect, if two sequences of
key parts are actually parts of tuples. In such a case
key_compare_parts does additional comparison by primary key parts,
if a NULL was met.

This parameters is actually a crutch, put by me to avoid code
doubling in tuple_compare_sequential_nullable. And this parameter
complicates #2988, where a sequential nullable tuple contains
optional parts.

Needed for #2988