Consider whether to add nullability annotations

[QuintinWillison]

In particular for our public APIs, but would also assist internally in some places too.

This section in the Java interoperability section of the Kotlin docs presents a nice summary of the options we could adopt.

I had previously asked sacOO7 to remove Jetbrains annotations when he added them in one of his PRs, but that was more because I didn't want to conflate this with that particular fix.

Working on this issue has three clear components:

1. Select which annotations to use
2. Add them to our public APIs
3. Possibly add them our internal APIs

┆Issue is synchronized with this Jira Research by Unito

[kavalerov]

Just to add my two cents - I am not sure about web and enterprise Java developers, but afaik in Android world de-facto standard is JetBrains annotations. Because of that I suggest we use them as well, unless there is a strong reason not to.

[sacOO7]

+1 @kavalerov . I can see it being used almost everywhere too. The only issue would be we need to configure it to generate an error via a plugin when it violates null contracts. Currently, IDE can only generate warnings when null constraints are violated.

[lawrence-forooghian]

I've just been creating an interface in our Asset Tracking Android product which is meant to wrap the ably-java SDK. I found myself having to use my knowledge of our SDKs in order to make judgements about nullability. If we had these annotations I wouldn't have needed to do that. All that is to say this sounds like a good idea 👍.