Only set store preferences if not null in head.html

web/templates/head.html

@@ -77,7 +77,9 @@
         window.mm_preferences = {{ .Preferences }};
 
         $(function() {
-            PreferenceStore.setPreferences(window.mm_preferences);
+            if (window.mm_preferences != null) {
+                PreferenceStore.setPreferences(window.mm_preferences);
+            }
         });
 
         if ({{.SessionTokenIndex}} >= 0) {