Ignore stripped lexemes in ts_rank_cd. Fixes #8354.

The cover density function requires positional information, and
ts_rank_cd would correctly handle tsvectors containing only stripped
lexemes by returning a rank of 0. However ts_rank_cd would produce a
non-zero rank when some matching lexemes were stripped and others not.

The problem is fixed by ignoring lexemes with no positional
information when generating the document representation used to
calculate cover density.