CollectionTables not properly parsing missing string values

[robertcv]

Downloading metadata with CollectionTables results in missing values being cast into a string object "nan" instead of the numpy/pandas object nan which may cause unexpected problems with further processing. This happens only for basic:string: fields. Additionally for basic:integer: and basic:decimal: fields we get pandas <NA> and numpy nan respectively which may also not be ideal and we should unify the not a number object we use.