Bug 1511345 Follow-up: Don't blow up on empty addonStartup.json r=zombie a=RyanVM

--HG--
extra : rebase_source : 104c6e59412aba85bc6206e9fd6ffa81df1e5892

toolkit/mozapps/extensions/internal/XPIProvider.jsm

@@ -1252,8 +1252,8 @@ var XPIStates = {
     // When upgrading from a build prior to bug 857456, convert startup
     // metadata.
     let done = false;
-    for (let location of Object.values(state)) {
-      for (let data of Object.values(location.addons)) {
+    for (let location of Object.values(state || {})) {
+      for (let data of Object.values(location.addons || {})) {
         if (!migrateAddonLoader(data)) {
           done = true;
           break;