[fix] allow nullable fileOrDir for client compatibility (flutter#8409)

The `flutter_enhancement_suite` uses `PubRoot.forDescendant` but does not ensure null safety. Since the plugin is not being actively maintained (but still adding value), this adjusts the API to be defensive.

Fixes flutter#8399

---

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Author: pq

src/io/flutter/pub/PubRoot.java

@@ -118,9 +118,13 @@ public static PubRoot forPsiFile(@NotNull PsiFile psiFile) {
    * Based on the filesystem cache; doesn't refresh anything.
    */
   @Nullable
-  public static PubRoot forDescendant(@NotNull VirtualFile fileOrDir, @NotNull Project project) {
+  public static PubRoot forDescendant(@Nullable VirtualFile fileOrDir, @NotNull Project project) {
+    // To be compatible w/ `flutter_enhancement_suite`, we allow a null `fileOrDir`.
+    // https://github.com/flutter/flutter-intellij/issues/8399
+    if (fileOrDir == null) return null;
     ProjectRootManager manager = ProjectRootManager.getInstance(project);
     if (manager == null) return null;
+    
     final ProjectFileIndex index = manager.getFileIndex();
     return OpenApiUtils.safeRunReadAction(() -> {
       final VirtualFile root = index.getContentRootForFile(fileOrDir);