[Feature] Add nullable reference annotations.

The playwright library has no clear documentation about whether an argument or return type can be null. In the original TypeScript library it’s not a problem since the type system of TypeScript supports nullable annotation. For example, the page.contentText has a string | null type, which indicates that it may be null. With strictNullCheck compiler option, you can’t call its methods without proper null check.

However, in this project, the nullable annotation is absent, neither the documentation points out the fact that it can be null.

Java actually has a plenty of null annotations. These annotations can also be identify in Kotlin. I hope nullable can be properly annotated like the TypeScript library, or at least be documented.

[3flex]

Couple of points:

1. From @yury-s: “we’d need to pick one of the third-party options which may work for some people but potentially cause conflicts for others” - is this really a blocker? There are countless projects adding nullability annotations of one flavour or another to their libraries, as far as I know the only major issue is that there’s no standardisation (JSpecify will hopefully one day address this, but until then…)
2. At least some nullability annotation would majorly assist all IDE users, and Kotlin users when compiling
3. If adding annotation to source is truly not an option, could we instead get external annotations? They can be published in a separate artifact.

[seekM]

Since Java 8 it’s possible to indicate that no value might be present using the Optional class. Could that maybe be used an alternative to annotations (for return types, at least)?