[iOS] JSON value '<null>' of type NSNull cannot be converted to NSString

• I have updated Purchases SDK to the latest version
• I have read the Contribution Guidelines
• I have searched the Community
• I have read docs.revenuecat.com
• I have searched for existing Github issues

Describe the bug
After Purchases.configure({ apiKey: REVENUECAT_API_KEY }); is called, Xcode console logs: JSON value '<null>' of type NSNull cannot be converted to NSString. Upon refreshing the simulator the error logs again. If I remove configure, the log is gone.

Update: I found that this error is triggered when appUserID is absent. https://github.com/RevenueCat/react-native-purchases/blob/85bba1e2c71cd5f05e798960fffa8d2ebfcaaf79/src/purchases.ts#L615
Not explicitly setting it to null (when absent) fixes it for me, so leaving it undefined.
https://github.com/RevenueCat/react-native-purchases/blob/85bba1e2c71cd5f05e798960fffa8d2ebfcaaf79/src/purchases.ts#L525

Now found that it was mentioned here that this warning can't be removed: https://github.com/RevenueCat/react-native-purchases/pull/1196
What is the downside of passing it as undefined to native instead of null? Which removes the warning...

1. Environment
   1. Platform: iOS
   2. SDK version:
   3. OS version: 26.5
   4. Xcode/Android Studio version: 26.6
   5. React Native version: 0.87.1
   6. SDK installation (CocoaPods + version or manual): 10.9.0
   7. Error on all iOS devices

Start with src/purchases.ts at the appUserID handling referenced around lines 525 and 615, then review pull request #1196 for the native-bridge context. Reproduce the warning by calling Purchases.configure without appUserID on iOS. Done means preserving configuration behavior while avoiding the NSNull-to-NSString warning when appUserID is absent.