Fix NPE in FileSystemProvider.

In case we can't get the contents of a directory.

Bug: 148836274
Test: No NPE
Change-Id: If896423f7788b614780cd7a07ffc75e25efc6ec5

core/java/com/android/internal/content/FileSystemProvider.java

@@ -430,7 +430,7 @@ public abstract class FileSystemProvider extends DocumentsProvider {
             if (shouldHide(file)) continue;
 
             if (file.isDirectory()) {
-                for (File child : file.listFiles()) {
+                for (File child : FileUtils.listFilesOrEmpty(file)) {
                     pending.add(child);
                 }
             }