Don't assert that paths object is present in asyncRequire

Summary:
Makes it possible to provide a `__loadBundleAsync` implementations in bundles that don't have `experimentalImportBundleSupport` (or the upcoming `lazy` flag) enabled, and thus don't serialise a `paths` object to pass to `asyncRequire` calls. React Native injects one on `main` since D43600052 so this is technically a fix - it prevents the non-lazy `import()` behaviour from breaking.

Changelog:
NOTE: Merge changelog entry with D43600050 and link to both commits.
* **[Feature]**: Support custom `__loadBundleAsync` implementations in the default `asyncRequire` function. See the [lazy bundling RFC](https://github.com/react-native-community/discussions-and-proposals/blob/main/proposals/0605-lazy-bundling.md) for more details.

Reviewed By: huntie

Differential Revision: D45161543

fbshipit-source-id: 8a9624b8ee0e083179ee03b5fa8f2760b6e79147

packages/metro-runtime/src/modules/asyncRequire.js

@@ -34,10 +34,12 @@ async function asyncRequireImpl(
 
   if (loadBundle != null) {
     const stringModuleID = String(moduleID);
-    const bundlePath = nullthrows(paths)[stringModuleID];
-    if (bundlePath != null) {
-      // NOTE: Errors will be swallowed by asyncRequire.prefetch
-      await loadBundle(bundlePath);
+    if (paths != null) {
+      const bundlePath = paths[stringModuleID];
+      if (bundlePath != null) {
+        // NOTE: Errors will be swallowed by asyncRequire.prefetch
+        await loadBundle(bundlePath);
+      }
     }
   }
 
@@ -68,11 +70,3 @@ asyncRequire.prefetch = function (
 };
 
 module.exports = asyncRequire;
-
-// Inline definition to save on a dependency
-function nullthrows<T>(value: ?T): T {
-  if (value == null) {
-    throw new Error('Unexpected null or undefined');
-  }
-  return value;
-}