Cleanup unnecessary null comparison before instanceof check in java.naming

Update code checks both non-null and instance of a class in java.naming module classes.
The checks and explicit casts could also be replaced with pattern matching for the instanceof operator.
For example the following code:

        return (obj != null &&
                obj instanceof CompoundName &&
                impl.equals(((CompoundName)obj).impl));


Can be simplified to:
    
        return (obj instanceof CompoundName other) &&
                impl.equals(other.impl);