fix(ios): remove optional from RequestPurchaseResult default value

The Promise<RequestPurchaseResult> return type requires a non-optional
value, but defaultResult was typed as RequestPurchaseResult? causing
a type mismatch in the Promise.async closure.

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

Author: hyochan

ios/HybridRnIap.swift

@@ -150,7 +150,7 @@ class HybridRnIap: HybridRnIapSpec {
 
     func requestPurchase(request: NitroPurchaseRequest) throws -> Promise<RequestPurchaseResult> {
         return Promise.async {
-            let defaultResult: RequestPurchaseResult? = .fourth([])
+            let defaultResult: RequestPurchaseResult = .fourth([])
             RnIapLog.payload(
                 "requestPurchase", [
                     "hasIOS": request.ios != nil,