bug#58862: 29.0.50; tab-bar-mode doesn't display after display-time-mode with format option

[Juri Linkov]

> Severity: minor
>
> In emacs -Q:
>
>     0. Evaluate
>            (setq tab-bar-format '(tab-bar-format-global))
>     1. M-x tab-bar-mode RET
>     2. M-x display-time-mode RET
>
> The tab bar is not visible.  I expect that it is, just as when I do
> this:
>
>     (progn
>        (setq tab-bar-format '(tab-bar-format-global))
>        (tab-bar-mode 1)
>        (display-time-mode 1))

The tab bar is not visible because an empty tab bar can't be displayed:

  (tab-bar-format-list tab-bar-format)
  => ((global menu-item "" ignore))

So it works fine when you change the initialization order to:

    0. Evaluate
           (setq tab-bar-format '(tab-bar-format-global))
    1. M-x display-time-mode RET
    2. M-x tab-bar-mode RET