Enable nullable: System.Management.Automation.Host.IHostSupportsInteractiveSession (#14170)

src/System.Management.Automation/engine/hostifaces/MshHost.cs

@@ -290,6 +290,7 @@ namespace System.Management.Automation.Host
     /// This interface needs to be implemented by PSHost objects that want to support the PushRunspace
     /// and PopRunspace functionality.
     /// </summary>
+#nullable enable
     public interface IHostSupportsInteractiveSession
     {
         /// <summary>
@@ -317,6 +318,6 @@ namespace System.Management.Automation.Host
         /// Returns the current runspace associated with this host.
         /// </summary>
         [SuppressMessage("Microsoft.Naming", "CA1704:IdentifiersShouldBeSpelledCorrectly", MessageId = "Runspace")]
-        Runspace Runspace { get; }
+        Runspace? Runspace { get; }
     }
 }