`floating_panes` not instantiated in new tabs when inside `default_tab_template`

[Stenodyon]

Issue description

When there is a floating_panes layout node inside the default_tab_template, floating panes will not be created in any new tabs, only in the first tab which is not the expected behavior as described in

There is a special default_tab_template node that can be used just like a regular tab_template node, but that would apply to all tabs in the template as well as all new tabs opened in the session.

(Emphasis mine.)

Minimal reproduction

layout {
    default_tab_template {
        floating_panes {
            pane name="floating"
        }
        pane name="not-floating"
    }
    tab
}

After creating a new tab, no floating pane is created, and toggling floating panes will spawn a new one (thus its name will not be "floating" as set in this example).

Other relevant information

Reproduced on master branch (at the time c72f3a712bfa92a4a80b4c1ad1dbe7669892a324).