Without a `else` block, a ConditionalBlock will always execute the first block

I think it comes from the fact that we take the last block, which is also the first when there's no `else` block.
https://github.com/tigerlily/solid/blob/96c98f512b8b1c67daa724db4644f05f674c038e/lib/solid/conditional_block.rb#L13