Turn on null strict comparison check for xplat js

Summary:
X-link: react/react-native#53379

X-link: react/react#34240

Changelog: [Internal]

Reviewed By: SamChou19815

Differential Revision: D80648362

fbshipit-source-id: cc47ae207f29a3ddb68bc0e029b8773f89503c52

Author: marcoww6

packages/metro-file-map/src/lib/RootPathUtils.js

@@ -57,6 +57,8 @@ export class RootPathUtils {
     const rootDirnames = [];
     for (
       let next = rootDir, previous = null;
+      /* $FlowFixMe[invalid-compare] Error discovered during Constant Condition
+       * roll out. See https://fburl.com/workplace/5whu3i34. */
       previous !== next;
       previous = next, next = path.dirname(next)
     ) {