String dtype: fix convert_dtypes() to convert NaN-string to NA-string
At the moment, when having a DataFrame/Series with the new (future) default string dtype, and calling `convert_dtypes()`, it would only convert object dtype with strings to the nullable (NA) string dtype, but not columns that already are the new string dtype. Similarly like default int64 or float64 columns are converted to Int64/Float64, also str should be converted to string I think.

xref https://github.com/pandas-dev/pandas/issues/54792
Thanks @jorisvandenbossche 