fix(NPE): fix NPE at VarCache.java.

Author: Enuviel

AndroidSDK/src/com/leanplum/internal/VarCache.java

@@ -520,7 +520,7 @@ public static void applyVariableDiffs(
         String name = entry.getKey();
         Map<String, Object> messageConfig = CollectionUtil.uncheckedCast(VarCache.messages.get
             (name));
-        if (messageConfig.get("action") != null) {
+        if (messageConfig != null && messageConfig.get("action") != null) {
           Map<String, Object> actionArgs =
               CollectionUtil.uncheckedCast(messageConfig.get(Constants.Keys.VARS));
           new ActionContext(