[CQ] fix nullability problems for flutter/pub (flutter#8306)

Fix nullability problems in `src/io/flutter/pub/`.

See flutter#8291.

If we pursue flutter#8292, we
could mark `src/io/flutter/pub/` as null-"clean".


---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

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  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide]([https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview)
for general expectations for PRs.
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Author: pq

flutter-idea/src/io/flutter/pub/PubRoot.java

@@ -177,7 +177,7 @@ public String getRelativePath(@NotNull VirtualFile file) {
     return path.substring(root.length() + 1);
   }
 
-  private static final String /*@NotNull*/[] TEST_DIRS = new String[]{ // TODO 2022.1
+  private static final String @NotNull [] TEST_DIRS = new String[]{ // TODO 2022.1
     "/test/",
     "/integration_test/",
     "/test_driver/",
@@ -237,6 +237,7 @@ public VirtualFile getPubspec() {
    */
   public boolean declaresFlutter() {
     validateUpdateCachedPubspecInfo();
+    assert cachedPubspecInfo != null;
     return cachedPubspecInfo.declaresFlutter();
   }
 
@@ -444,7 +445,10 @@ public Module getModule(@NotNull Project project) {
     if (project.isDisposed()) {
       return null;
     }
-    return ProjectRootManager.getInstance(project).getFileIndex().getModuleForFile(pubspec);
+    var manager = ProjectRootManager.getInstance(project);
+    if (manager == null) return null;
+
+    return manager.getFileIndex().getModuleForFile(pubspec);
   }
 
   @Override