fix nullability annotations in authUI

[morganchen12]

Fixes #99.

[asciimike]

Mostly since I'm curious...

What's the difference between:

- fooWithBar:(nullable Bar *)bar;

And:

- fooWithBar:(Bar *_Nullable)bar;

Looks like nothing: http://stackoverflow.com/questions/32452889/difference-between-nullable-nullable-and-nullable-in-objective-c

Also, looks like the Google Objective-C style guide is missing guidance on nullability.

I prefer the first (and would like we use them everywhere we're allowed to, aka everywhere the compiler doesn't throw a fit [typedefs and blocks have special rules]).

[morganchen12]

They're the same. There's also a third:

- (void)fooWithBar:(Bar *__nullable)bar;

The postfix _Nullable can be more useful(?) when referring to references of references, like:

- (void)fooWithBar:(Bar *)bar error:(NSError *_Nullable *_Nonnull)error;

but it's such a rare case that it probably doesn't matter. We should just pick one and commit to it (and update the Google style guide eventually). The prefix version definitely looks nicer.

[asciimike]

Cool, thanks for the clarification :)