Add nullability annotations for RMQTLSOptions class

[sasikiran]

In the following statement,

RMQTLSOptions *tlsOptions = [RMQTLSOptions fromURI:urlString verifyPeer:NO];

the urlString was accidentally nil and the app crashed. When I checked the code, I found that the class has missing nullability annotations. So, I added these.

[camelpunch]

Thanks for this. Could you please add a test to reproduce the issue? I can help or do this for you if the tests are a bit daunting.

[camelpunch]

Apologies, that comment ignored the fact that this is a compile-time change. Thanks for this!

[sasikiran]

You are right. It's a compiler level feature and hence I couldn't write a test. Thank you.