box: fix unique violation in functional index with nullable parts

Currently is_nullable property of a functional index part disables the
unique property of the index. The bug is in func_index_compare(), which
compares functional keys first, and if they are equal it compares the
primary keys. This behaviour is correct only when some part of the key
is NULL (and for non-unique indexes), but for now the primary keys are
compared unconditionally. Fix this by checking for NULL key parts.

Closes #8587

NO_DOC=bugfix