String dtype: coerce missing values in indexers for string dtype Index
Closes #59879

Re-taking part of the changes in https://github.com/pandas-dev/pandas/pull/60329 that I had removed from that PR to let that just focus on fixing looking up its "native" missing value for string dtype. This PR then relaxes that restriction to allow looking up other missing values as well.


lgtm. I don't think the macOS failures are related? Thanks @jorisvandenbossche 