Fix NullPointerException in IterableInAppManager.syncWithRemoteQueue

Add null safety checks to prevent NPE caused by race conditions where
messages may be null or removed from storage between check and access.
Consolidates the duplicate storage lookup into a single call and adds
null guards when iterating both remote and local message queues.

Fixes #485

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: franco-zalamena-iterable

iterableapi/src/main/java/com/iterable/iterableapi/IterableInAppManager.java

@@ -339,20 +339,20 @@ private void syncWithRemoteQueue(List<IterableInAppMessage> remoteQueue) {
         Map<String, IterableInAppMessage> remoteQueueMap = new HashMap<>();
 
         for (IterableInAppMessage message : remoteQueue) {
+            if (message == null) {
+                continue;
+            }
+
             remoteQueueMap.put(message.getMessageId(), message);
 
-            boolean isInAppStored = storage.getMessage(message.getMessageId()) != null;
+            IterableInAppMessage localMessage = storage.getMessage(message.getMessageId());
 
-            if (!isInAppStored) {
+            if (localMessage == null) {
                 storage.addMessage(message);
                 onMessageAdded(message);
 
                 changed = true;
-            }
-
-            if (isInAppStored) {
-                IterableInAppMessage localMessage = storage.getMessage(message.getMessageId());
-
+            } else {
                 boolean shouldOverwriteInApp = !localMessage.isRead() && message.isRead();
 
                 if (shouldOverwriteInApp) {
@@ -363,7 +363,11 @@ private void syncWithRemoteQueue(List<IterableInAppMessage> remoteQueue) {
             }
         }
 
-        for (IterableInAppMessage localMessage : storage.getMessages()) {
+        List<IterableInAppMessage> localMessages = storage.getMessages();
+        for (IterableInAppMessage localMessage : localMessages) {
+            if (localMessage == null) {
+                continue;
+            }
             if (!remoteQueueMap.containsKey(localMessage.getMessageId())) {
                 storage.removeMessage(localMessage);