Fix crash in OneSignalAttachmentHandler trimURLSpacing

[nan-li]

Description

One Line Summary

Fix crash in OneSignalAttachmentHandler trimURLSpacing by checking for NSNull in addition to nil checking that is already existent.

Details

The attachment handler can have values in a dictionary be NSNull since is used to represent nil objects in collections, and we typically see NSNull when we convert JSON to Objective-C objects and the JSON contained null values.

Motivation

• Fixes https://github.com/OneSignal/OneSignal-iOS-SDK/issues/1400

Scope

Checking for nullness

Testing

Unit testing

Added one unit test

Manual testing

Reproduced the crash by passing in NSNull manually. Tested on iPhone 13 ios 17.4 by attaching an image and seeing method triggered.

Affected code checklist

• [ ] Notifications
  • [ ] Display
  • [ ] Open
  • [ ] Push Processing
  • [ ] Confirm Deliveries
• [ ] Outcomes
• [ ] Sessions
• [ ] In-App Messaging
• [ ] REST API requests
• [ ] Public API changes

Checklist

Overview

• [x] I have filled out all REQUIRED sections above
• [x] PR does one thing
• [ ] Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• [x] I have included test coverage for these changes, or explained why they are not needed
• [x] All automated tests pass, or I explained why that is not possible
• [x] I have personally tested this on my device, or explained why that is not possible

Final pass

• [x] Code is as readable as possible.
• [x] I have reviewed this PR myself, ensuring it meets each checklist item

---

This change is