refactor: improve robustness of `ReadConcern.fromOptions`
refactor: improve robustness of `ReadConcern.fromOptions`

We made some assumptions in this method about the existence of
keys, as well as that no `readConcern` passed in would ever already
be of the type we were looking to cast to.

mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native,mongodb/node-mongodb-native