This PR fixes #12247

Internally, the parser rewrites unless blocks:

{{ unless { the_var } }}
    ...
{{ /unless }}

into its corresponding if statement:

{{ if ! ({ the_var }) }}
    ...
{{ /if }}

After investigation, it was discovered that the interpolated regions produced by the nested { the_var } syntax was not being copied to the rewritten node. This PR copies this information, allowing the unless node to work correctly in this situation.