[ngrams] ngramsValidPatch put to appropriate use

For ngrams table, we set ngramsValidPatch = mempty because the table
is refreshed and we have issue
#594

However, we need to keep that field, it turns out, because we have
#598

Recently applied patches are needed for the document ngrams
highlighting.

This all would be unnecessary (or at least moved to the backend) if we
had
#553