Update to how we handle null or undefined Suspense fallbacks

[rickhanlonii]

Overview

We've made a change to the way nullish Suspense fallbacks are handled in the latest alpha. tl;dr: We don't skip Suspense boundaries with missing or null fallbacks any more. Instead, we capture the boundary and render null for the fallback.

Background

Previously, if you had a Suspense boundary that didn't provide a fallback, we would silently skip it and continue searching upwards for the next boundary:

<Suspense fallback={<Loading />}>   // <--- this boundary is used
  <Suspense>                        // <--- this boundary is skipped, no fallback
    <Page />
  </Suspense>
</Suspense>

We found that this can result in confusing, hard to debug situations. For example, if you're debugging an issue and throw in a quick boundary without a fallback to test an issue, it won't change the behavior of the app as you may expect and there's no warning that it's missing a fallback.

Change

Now, we will use the Suspense boundary, even if the fallback is null or undefined:

<Suspense fallback={<Loading />}>   // <--- not used
  <Suspense>                        // <--- this boundary is used, rendering null for the fallback
    <Page />
  </Suspense>
</Suspense>

This change means that we're no longer silently skipping boundaries. Instead, we'll capture at the boundary and render the fallback as if you provided a component that returned null. This means the semantics of suspending will work as expected, and if you forget to provide a fallback then you'll see the null loading state.