Added correct nullability values for initializers

[clementpadovani]

• I’ve read and understood the Contributing guidelines and have done my best effort to follow them.
• I’ve read and agree to the Code of Conduct.
• I’ve been mindful about doing atomic commits, adding documentation to my changes, not refactoring too much.
• I’ve a descriptive title and added any useful information for the reviewer. Where appropriate, I've attached a screenshot and/or screencast (gif preferrably).
• I’ve written tests to cover the new code and functionality included in this PR.
• I’ve read, agree to, and signed the Contributor License Agreement (CLA).

PR Summary

Based on the documentation, I’ve added nullability qualifiers.

Related Issues

N/A

Test strategy

N/A

[dzenbot]

Looks good. Thanks!

[wlisac]

@clementpadovani @dzenbot The comment says these initializers may return nil, but looking at the implementation, I don't see why these should be nullable.

- (instancetype)initWithTableViewStyle:(UITableViewStyle)style
{
    NSAssert([self class] != [SLKTextViewController class], @"Oops! You must subclass SLKTextViewController.");
    NSAssert(style == UITableViewStylePlain || style == UITableViewStyleGrouped, @"Oops! You must pass a valid UITableViewStyle.");

    if (self = [super initWithNibName:nil bundle:nil])
    {
        self.scrollViewProxy = [self tableViewWithStyle:style];
        [self slk_commonInit];
    }
    return self;
}

The super designated initializer is not nullable / failable: [super initWithNibName:nil bundle:nil]

Maybe I'm missing something, but it would be nice if we could make these non-failable initializers 🙂