Anim: fix crash when writing empty action groups

It was just a missing 'else' clause around some should-have-been-
conditional code.

Pull Request: https://projects.blender.org/blender/blender/pulls/127241

source/blender/blenkernel/intern/action.cc

@@ -390,7 +390,9 @@ static void action_blend_write_make_legacy_channel_groups_listbase(
     if (fcurves.is_empty()) {
       group->channels = {nullptr, nullptr};
     }
-    group->channels = {fcurves.first(), fcurves.last()};
+    else {
+      group->channels = {fcurves.first(), fcurves.last()};
+    }
   }
 
   /* Determine the prev/next pointers on the elements. */