Bug 1701234 - Add more null check on the background thread. r=geckoview-reviewers,agi

Even if landing bug 1697683, this NullPointerException occurs on the
background thread. So I would like to add more null check for this crash.

Differential Revision: https://phabricator.services.mozilla.com/D132329

mobile/android/geckoview/src/main/java/org/mozilla/gecko/SpeechSynthesisService.java

@@ -65,6 +65,10 @@ public class SpeechSynthesisService {
           @Override
           public void run() {
             final TextToSpeech tss = getTTS();
+            if (tss == null) {
+              Log.w(LOGTAG, "TextToSpeech is not initialized");
+              return;
+            }
             final Locale defaultLocale =
                 Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR2
                     ? tss.getDefaultLanguage()