bug#63759: 29.0.91; dockerfile-ts-mode--imenu returns invalid index

[Eli Zaretskii <eliz@gnu.org>]

> From: Jimmy Yuen Ho Wong <wyuenho@gmail.com>
> Date: Sat, 27 May 2023 18:38:29 +0100
> 
> 
> Reproduction:
> 
> 0. Open a Emacs GUI, i.e. NS port
> 1. C-x C-f /some/nonexistent/Dockerfile
> 2. M-x toggle-debug-on-error
> 2. M-x imenu-add-menubar-index
> 3. Click on the Index > Stage menu item
> 4. Observe the following error pops up
> 
> Expectation:
> 
> If the dockerfile-ts-mode buffer is empty, there should not be an extra
> item preset in the imenu index that when clicked on will jump to a
> non-existent location and result in an error.
> 
> ```
> Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
>   imenu-default-goto-function("Stage" nil)
>   imenu(("Stage" nil imenu-default-goto-function))
>   imenu(("Stage"))
>   imenu--menubar-select(("Stage"))
>   #f(compiled-function () (interactive nil) #<bytecode 0xcfe8182542f4cbd>)()
>   funcall-interactively(#f(compiled-function () (interactive nil) #<bytecode 0xcfe8182542f4cbd>))
>   #<subr call-interactively>(#f(compiled-function () (interactive nil) #<bytecode 0xcfe8182542f4cbd>) nil nil)
>   call-interactively@ido-cr+-record-current-command(#<subr call-interactively> #f(compiled-function () (interactive nil) #<bytecode 0xcfe8182542f4cbd>) nil nil)
>   apply(call-interactively@ido-cr+-record-current-command #<subr call-interactively> (#f(compiled-function () (interactive nil) #<bytecode 0xcfe8182542f4cbd>) nil nil))
>   call-interactively(#f(compiled-function () (interactive nil) #<bytecode 0xcfe8182542f4cbd>) nil nil)
>   command-execute(#f(compiled-function () (interactive nil) #<bytecode 0xcfe8182542f4cbd>))
> ```

Thanks.

Randy, can you p-lease look into this?