TYP: make na_value consistently a property
Similar to `_engine_type`, `na_value` is sometimes a property and sometimes a class variable.

There were multiple places that access `StringDtype.na_value` (works with class variables but not properties). I replaced these cases with the value of `StringDtype().na_value`.


(and one `_engine_type` class variable I forgot in another PR)
Thanks @twoertwein 