fix: NotEqualTo trigger no longer matches non-existent properties

- Changes the behavior of trigger evaluation when a property doesn't exist.
- Previously, a trigger with NotEqualTo operator would return true for non-existent properties (e.g., subscription_type != "premium" would match users without any subscription_type set).
- Now, only the NotExists operator returns true for non-existent properties. NotEqualTo requires the property to exist with a different value.
- This provides stricter trigger matching where NotEqualTo only matches users who have explicitly set a value that differs from the trigger value.

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalInAppMessages/Controller/OSTriggerController.m

@@ -181,11 +181,9 @@ - (BOOL)messageMatchesTriggers:(OSInAppMessageInternal *)message {
 - (BOOL)evaluateTrigger:(OSTrigger *)trigger forMessage:(OSInAppMessageInternal *)message {
     if (!self.triggers[trigger.property] && [trigger.kind isEqualToString:OS_DYNAMIC_TRIGGER_KIND_CUSTOM]) {
         // The value doesn't exist
-        
-        // The condition for this trigger is true since the value doesn't exist
-        // Either loop to the next condition, or return true if we are the last condition
-        return trigger.operatorType == OSTriggerOperatorTypeNotExists ||
-        (trigger.value && trigger.operatorType == OSTriggerOperatorTypeNotEqualTo);
+
+        // Only NotExists operator should return true for non-existent values
+        return trigger.operatorType == OSTriggerOperatorTypeNotExists;
 
     } else if (trigger.operatorType == OSTriggerOperatorTypeExists) {
         return true;