[iagno] Add cast to fix build with --enable-experimental-non-null

[Rico Tzschichholz <ricotz src gnome org>]

commit 49a7f4873d2cafcf86a649ad041bcec083401d34
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Thu Feb 28 16:16:40 2019 +0100

    Add cast to fix build with --enable-experimental-non-null

 src/game.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/game.vala b/src/game.vala
index 4006bbc..904c7b5 100644
--- a/src/game.vala
+++ b/src/game.vala
@@ -157,7 +157,7 @@ private class Game : Object
 
         current_color = to_move;
 
-        warn_if_fail (string.joinv ("\n", setup).strip () == to_string ().strip ());
+        warn_if_fail (string.joinv ("\n", (string?[]) setup).strip () == to_string ().strip ());
     }
 
     internal string to_string ()