Add more error checking when creating User objects

[cmelchior]

An attempt at finding https://github.com/realm/realm-java/issues/7847

Observations:

• We are never setting the osUser reference to null, perhaps it is being passed in as such? But all the code paths we have for that look safe enough.
• We are only overriding the osUser ref when calling linkCredentials, but I don't see how that can nullify it either.

[cmelchior]

The real cause was discovered and fixed in https://github.com/realm/realm-java/pull/7849