The current API returns a Handle<NativeContext> which can be
optionally null and all the users of this API never actually
checked for this null value.

Previously, this wasn't a problem as all the possible JSObjects
that were user visible would return a valid NativeContext but now
there are wasm objects that don't have a valid constructor so don't
have a NativeContext.

Bug: v8:11451, chromium:1166077
Change-Id: I4fd5edf8f1a750e6f0abb931fd41358e5ae4dfcf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2692695
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72769}