Fix build-break in GBS/OBS build

In MCS, Operator `??' cannot be applied to operands of type `string' and `string'

Change-Id: Ic945fb2260065df0059df5940946fe48490d51a0

diff --git a/Tizen.Xamarin.Forms.Extension/RadioButton.cs b/Tizen.Xamarin.Forms.Extension/RadioButton.cs
index 20b30db..f2e4f23 100755 (executable)
--- a/Tizen.Xamarin.Forms.Extension/RadioButton.cs
+++ b/Tizen.Xamarin.Forms.Extension/RadioButton.cs
@@ -114,7 +114,7 @@ namespace Tizen.Xamarin.Forms.Extension
         public string GroupName
         {
             get { return (string)GetValue(GroupNameProperty); }
-            set { SetValue(GroupNameProperty, value ?? default(string)); }
+            set { SetValue(GroupNameProperty, value); }
         }
 
         /// <summary>
@@ -176,4 +176,4 @@ namespace Tizen.Xamarin.Forms.Extension
             radioButton.Selected?.Invoke(radioButton, new SelectedEventArgs((bool)newValue));
         }
     }
-}
\ No newline at end of file
+}