Update signature of Map.computeIfAbsent

[msridhar]

mappingFunction is allowed to return null to avoid updating the mapping, and in this case, computeIfAbsent returns null. Also updated overrides in @NullMarked classes.

I added the @PolyNull comment on Map.computeIfAbsent since I think we'd use it here if it were available; didn't add the comment on all the overrides.

Discovered in ben-manes/caffeine#2004 (comment)

[cpovirk]

That is all true, yet this has been the only(?) case in which I've been holding out hope that we can declare the signature as more restrictive than it really "should" be, just because a @Nullable return type is so inconvenient for 99% of callers. There is some discussion in #102, but beware that the actual title of that issue is about going even further in the opposite direction (as probably no one actually wants), even though the text goes on to mostly discuss the thing proposed here that people reasonably would want :) I'd be interested in your read on it, whether here or there. I do kind of feel like we'll need to go with the "honest" type eventually, though it would be nice to sort out the Kotlin story first, and I haven't found time :(

[msridhar]

Ah...bummer. We could change the type of mappingFunction to be Function<? super K, ? extends @Nullable V>, but (inconsistently) leave the return type as V (without a @Nullable). But the current version is safer.

We have an open issue (uber/NullAway#1616) on adding some kind of ad hoc @PolyNull support to NullAway for library models, for orElseGet. We might have to do the same thing here, or just ask users to suppress the warning. I'll have to think about how best to recommend a narrow suppression, though, rather than the entire surrounding method.