fix(dynamic): respect parent Suspense boundaries when no loading component is provided

[MD-Mushfiqur123]

What?

Fixes next/dynamic() creating an unnecessary inner <Suspense> boundary with an empty fallback when no loading component is provided, which prevents parent <Suspense> boundaries from working and causes layout flicker.

Why?

When using next/dynamic() without a loading component inside a parent <Suspense>:

<Suspense fallback={<FullPageSkeleton />}>
  <DynamicComponent /> {/* no loading prop */}
</Suspense>

The current code forces an inner <Suspense> boundary when ssr: false (line 52: const hasSuspenseBoundary = !opts.ssr || !!opts.loading). This inner boundary has fallback={null}, which:

1. Catches the lazy suspension before it reaches the parent <Suspense>
2. Renders nothing during loading, causing a visible layout flicker
3. Makes the parent fallback completely useless

How?

lazy-dynamic/loadable.tsx: Changed the condition so that a <Suspense> boundary is only created when a loading component is explicitly provided. When loading is null (default), use <Fragment> instead, allowing any parent <Suspense> to properly handle the loading state.

loadable.shared-runtime.tsx: Added a null guard for opts.loading to prevent a React.createElement(null, ...) crash when no loading component is provided.

Verification

• When loading=<Component>: <Suspense> is used with the given fallback (unchanged behavior)
• When loading=null (default) with ssr=true: <Fragment> is used, parent <Suspense> catches the lazy state
• When loading=null (default) with ssr=false: <Fragment> is used, <BailoutToCSR> triggers the parent <Suspense>

Fixes #73830

[vercel]

When ssr: false and no loading component is provided to next/dynamic(), the Suspense boundary is removed, causing BailoutToCSRError to propagate as a fatal SSR error instead of gracefully bailing out to client-side rendering.