Bug 1289006 - Return gracefully if listFiles returns null in telemetry store. r=grisha

This changeset will correct the crash we're seeing in the bug.

The docs support that File.listFiles can return null.

MozReview-Commit-ID: FHYGErshhoP

--HG--
extra : rebase_source : 15d0c4a3d283924627f1f97a1f99637244c49c08

mobile/android/base/java/org/mozilla/gecko/telemetry/stores/TelemetryJSONFilePingStore.java

@@ -129,7 +129,15 @@ public class TelemetryJSONFilePingStore extends TelemetryPingStore {
 
     @Override
     public ArrayList<TelemetryPing> getAllPings() {
-        final List<File> files = Arrays.asList(storeDir.listFiles(uuidFilenameFilter));
+        final File[] fileArray = storeDir.listFiles(uuidFilenameFilter);
+        if (fileArray == null) {
+            // Intentionally don't log all info for the store directory to prevent leaking the path.
+            Log.w(LOGTAG, "listFiles unexpectedly returned null - unable to retrieve pings. Debug: exists? " +
+                    storeDir.exists() + "; directory? " + storeDir.isDirectory());
+            return new ArrayList<>(1);
+        }
+
+        final List<File> files = Arrays.asList(fileArray);
         Collections.sort(files, fileLastModifiedComparator); // oldest to newest
         final ArrayList<TelemetryPing> out = new ArrayList<>(files.size());
         for (final File file : files) {