fix: SDKE-627 Return true immediately for isInitialized on iOS

Partners receive null responses because Flutter SDK wasn't fully initialized.
Changed isInitialized to return true immediately to allow SDK usage without waiting.

Author: denischilik

ios/Classes/SwiftMparticleFlutterSdkPlugin.swift

@@ -26,7 +26,7 @@ public class SwiftMparticleFlutterSdkPlugin: NSObject, FlutterPlugin {
   public func handle(_ call: FlutterMethodCall, result: @escaping FlutterResult) {
     switch call.method {
     case "isInitialized":
-        result(MParticle.sharedInstance().initialized)
+        result(true)
     case "getAppName":
         result(Bundle.main.infoDictionary?[kCFBundleNameKey as String] as? String)
     case "getOptOut":