Don't use invalid defaults for loadouts (#28740)

* Don't use invalid defaults for loadouts

At the time it made more sense but now with species specific stuff it's better to have nothing.

* Loadout SetDefault only applies valid loadouts

Content.Shared/Preferences/Loadouts/Effects/LoadoutEffect.cs

@@ -13,7 +13,7 @@ public abstract partial class LoadoutEffect
     public abstract bool Validate(
         HumanoidCharacterProfile profile,
         RoleLoadout loadout,
-        ICommonSession session,
+        ICommonSession? session,
         IDependencyCollection collection,
         [NotNullWhen(false)] out FormattedMessage? reason);