logging out of anonymous user is a no-op

* As discussed in design forum, calling logout multiple times should result in a no-op.
* Note that states can carry over.

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalUser/Source/OneSignalUserManagerImpl.swift

@@ -322,10 +322,14 @@ public class OneSignalUserManagerImpl: NSObject, OneSignalUserManager {
     }
 
     /**
-     The SDK needs to have a user at all times, so this method will create a new anonymous user.
+     The SDK needs to have a user at all times, so this method will create a new anonymous user. If the current user is already anonymous, calling `logout` results in a no-op.
      */
     @objc
     public func logout() {
+        guard user.identityModel.externalId != nil else {
+            OneSignalLog.onesignalLog(.LL_DEBUG, message: "OneSignal.User logout called, but the user is currently anonymous, so not logging out.")
+            return
+        }
         prepareForNewUser()
         _user = nil
         createUserIfNil()