fix: string.Empty causes error in BankId (#519)

Update default returnUrl from String.Empty to null to comply with new BankID requirements and prevent 400 errors

Author: Zonnex

src/ActiveLogin.Authentication.BankId.Core/Launcher/BankIdLauncher.cs

@@ -170,12 +170,10 @@ private static string GetIOsBrowserSpecificRedirectUrl(BankIdSupportedDevice dev
             BankIdSupportedDeviceBrowser.Chrome => IosChromeScheme,
             BankIdSupportedDeviceBrowser.Firefox => IosFirefoxScheme,
 
-            // Opens a new tab on app launch, so can't launch automatically
-            BankIdSupportedDeviceBrowser.Edge => string.Empty,
-            BankIdSupportedDeviceBrowser.Opera => string.Empty,
+            BankIdSupportedDeviceBrowser.Edge => NullRedirectUrl,
+            BankIdSupportedDeviceBrowser.Opera => NullRedirectUrl,
 
-            // Return empty string so user can go back manually, will catch unknown third party browsers
-            _ => string.Empty
+            _ => NullRedirectUrl
         };
     }