Fix string NA vs NA in MANC for cf_meta

[mmc46]

In MANC, there are string NAs, instead of actual NA (there are also "" that I've come across).
Could the string NAs and empty strings be replaced by NA so the output is already cleaned up?

 cf_ids('/type:(pIP10|pMP2).*',
datasets = c("manc", "malecns", "yakubavnc")))

testna |> count(type, lineage, dataset)
   type lineage   dataset n
1 pIP10      NA      manc 2
2 pIP10    <NA>   malecns 2
3 pIP10    <NA> yakubavnc 2
4  pMP2      NA      manc 2
5  pMP2    <NA>   malecns 2
6  pMP2    <NA> yakubavnc 2